(FORTUNE Magazine) – While casinos will warmly welcome busloads of blue-haired old ladies aching to play the one-armed bandits, they lavish most of their attention on a handful of high rollers. And no wonder. Nicknamed "whales" for their $100,000-plus bets and multimillion-dollar lines of credit, these gamblers typically account for 20% of a casino's winnings.

Most whales are male entrepreneurs, over 40, and from Pacific Rim countries. Their preferred games: baccarat and blackjack. It's a select group--some 250 worldwide--and rivalry for their business is fierce. Casinos like the Mirage and the Hilton have marketing staffs that scout potential Moby Dicks in other casinos and at upscale charity events abroad.

The casinos snare these high rollers with amenities like first-class transportation and ultra-lush accommodations. Last year the Las Vegas Hilton completed three penthouse villas for their "premium players," those with $1 million-plus credit lines. The villas, which cost $40 million to build, each have a swimming pool, media room, and 24-hour butlers trained by the man who also educates butlers for England's royal family.

How much does such service cost the whales? Absolutely nothing. As the casinos say, they're "comped." All a high roller has to do is lose a million every now and then. The kind of freebies a casino will give a high roller depends on his betting history and the number of visits to a property. For example, a player who wagers $50,000 a hand and visits the Mirage six times a year may get one of its eight luxury apartments, while someone coming only twice a year will likely get a "very nice standard suite."

But courting the high end does involve elements of chance for casinos. Last year Kerry Packer, Australia's billionaire media tycoon, took the MGM Grand for $20 million at blackjack in only 40 minutes. Finding his play too costly, Packer was reportedly asked to gamble elsewhere, a request MGM's blackjack dealers likely regretted. The Australian is known as the King of "King Georges" (Vegas-ese for a big tipper). He once tipped a cocktail waitress by picking up the balance on her mortgage.

--Joe McGowan