(FORTUNE Magazine) – For a country with a tax-heavy, welfare-state economy, Holland has a surprising concentration of private wealth. Five Dutch families have blossomed to billionaire dimensions in the country's waterlogged soil. Why is the Netherlands such a hothouse for growing guilders? Mostly because these Dutch uncles tenaciously hold on to their family money and jealously guard their privacy. Though none of the billionaires trace their riches as far back as the 17th century, when Flying Dutchmen plied the seas of a great trading empire, all inherited their money. They nurtured 19th-century wealth built in mining, manufacturing, and retailing, and expanded it through two world wars and three generations. Says David A. Slager, a prominent Rotterdam attorney: ''Preserving wealth in family hands is the single-minded pursuit of the Dutch dynasties.'' Tracing the wealth of Holland's billionaires calls for an equally determined effort. The Dutch super-rich stash their hoards in family foundations that often disappear in a labyrinth of holding companies set up in the Netherlands Antilles, a haven from tax collectors. To determine the families that qualified as billionaires, FORTUNE relied in part on the sleuthing of Jos van Hezewijk, a scholar who tracks the holdings of rich Hollanders. ''Wealthy Dutchmen would rather talk about their sex lives than their money, and their sex lives are far less interesting,'' says van Hezewijk, who recently published a book titled The Top Elite of the Netherlands. His research, based on 300 interviews over four years, yields a fascinating roster of the renowned and the reclusive. Holland's best-known billionaire undoubtedly is Alfred ''Freddy'' Heineken, 63. The breweries that he inherited in 1971 had been in family hands for a century and dominated the small Dutch market. But it was Freddy who lifted Heineken from the plebeian ranks of Europe's hundreds of family-owned breweries into one of the world's best-known consumer franchises. In 1984 FORTUNE chose him as one of Europe's ablest managers. Like the other heirs to Dutch fortunes, Freddy Heineken went through a long apprenticeship before taking charge of the family business. In the late 1940s his father sent him to study American marketing techniques with Heineken's New York distributor. Freddy lived in a $3.50-a-night Times Square hotel room and peddled his beer bar to bar for three years. Says he: ''Anyone who doesn't know what it means to have painful feet doesn't know what selling is.'' The price that young Freddy paid in blisters and worn shoe leather yielded a yeasty return. Today Heineken dominates the U.S. imported beer market with a better-than-30% share, far ahead of its nearest rival, Mexico's Corona. Freddy Heineken's estimated 50% stake in the company is worth over $1.2 billion. His other holdings, which include properties in Cap d'Antibes and St. Moritz, a fleet of rare antique cars, and an extensive collection of modern art masterpieces, including several of Picasso's best-known later works, are probably worth another $100 million. Two of Holland's billionaire families, the Fentener van Vlissingens and the van Beuningens, secured their wealth through a mid-19th-century marriage. Frederik Fentener van Vlissingen and Hendrik Adriaan van Beuningen were two of Holland's leading coal traders. Together the families dominated Holland's coal cartel until the end of World War II, when the Fentener van Vlissingens bought out their van Beuningen in-laws. Today the Fentener van Vlissingens' $2.5 billion is in the hands of three brothers. Frederik, 54, controls the family holding company. Paul, 46, has $ turned the coal-trading outfit into a worldwide energy and consumer goods distribution company that ranks among the ten largest Dutch corporations. John, 58, manages the Noro Group, a leading Dutch investment fund. Their van Beuningen relations -- worth $2.4 billion -- have also thrived handsomely, if obscurely. Brothers Hendrik Adriaan van Beuningen, 71, Hendrik Jan Engelbert, 67, and their sister, Hendrike, 72, control a private investment fund, a shipping company, and a maritime salvage firm. The tie between business and family is seamless for most of the super-rich Dutch, but for the Brenninkmeyer clan the bond borders on obsessive. Owners of a 500-store clothing chain called C&A (after brothers Clemens and August, who got their start peddling 19th-century wares from farm to farm in northern Germany), the Brenninkmeyers are Europe's richest retailers. Although their C& A logo is a colorful standout, the family is cloaked in secrecy. Current family head Godfried Brenninkmeyer declines all requests for interviews and forbids family members to talk to the press. The company has no shareholders outside the family and publishes no financial statements. The family won't even disclose Godfried's age. Estimates place the family's worth at $3.4 billion and Godfried's age at 77. The dearth of fact about the Brenninkmeyer clan has spurred an abundance of legend. One: When the family council convenes at its Amsterdam headquarters, one chair remains vacant for onwe lieve heer (our dear Lord). Another: Would- be suitors must pass muster with the family council before any young Brenninkmeyer is wed. Another: When a family member dies in any part of the world, 100 guilders is given to the local Catholic priest to say a mass for the deceased. The family, of course, has no comment. Anton Dreesmann, 64, solidified another retailing fortune when he took command of his family's business 14 years ago. The Dreesmann clan had prospered for a century as owners of a chain of department stores called Vroom & Dreesmann. The business grew with the family. Each of the 60 stores was run by a manager drawn from the ever-widening family circle, with half of each store's profits going to the manager and half to the family company. But as the chain grew, problems developed, reaching a crisis in the early 1970s when consumer spending tapered off. Anton, who studied law and earned a doctorate in economics at Amsterdam University, joined the family business as a salesclerk in 1949 and later rescued one of its flagship stores in The Hague. Determined to consolidate the business under strong central management, he waited patiently for older family members to retire and bought the shares of relatives who were not actively involved in the stores. Firmly in control by the mid-1970s, Anton reorganized the company into 13 divisions and renamed it Vendex International. By diversifying into faster-growing businesses such as occupational training, property management, and a consumer credit-rating service, Dreesmann boosted sales from $1 billion in 1973 to $9 billion last year and tripled profits in the past five years to more than $150 million. He and his family are worth $2.4 billion. Dreesmann says he plans to take the company public by 1990. But he holds court behind a desk more like the landing deck of an aircraft carrier. And he still runs his empire like a family business. To illustrate he punches the intercom with a pudgy finger and asks a secretary to recite the company motto. Over the box a voice responds, ''Authority, responsibility, attention, and love.'' Anton leans back and beams. ''Those are the qualities that are lacking in most families today, but they are the essence of life.'' Founding a world-class business, grooming generations of family members to manage it, and sitting on the shares is Holland's unbeatable formula for creating billionaire-size wealth. The Philips family gave their name to Europe's largest electronics company and ran it until the mid-1970s, but they failed to make the cut of Dutch billionaires. In 1891, Gerard Philips, an engineer and inventor, opened a factory in an abandoned tannery to manufacture a light bulb he had perfected. He was on the verge of collapse when his 20- year-old brother, Anton, reluctantly agreed to join the company. He scoured Europe for orders and landed a contract to supply 50,000 bulbs for Czar Nicholas's Winter Palace in St. Petersburg. In part because the Czar wanted his light bulbs changed every year, Philips' Gloeilampenfabrieken prospered, eventually becoming a $22.5-billion-a-year giant. Though the continuity of family leadership -- first under Anton's son-in- law, Frans Otten, and then, until 1975, Anton's son Frits -- helped Philips survive the destruction of World War II and the postwar onslaught of Japanese competition, the Philips family's net worth is less than $50 million today. Anton turned out to be a better bulb salesman than an investor. He sold most of the family's shares in 1912, saying, ''I can't make objective business decisions if my personal wealth is tied up with the company.''