(FORTUNE Small Business) – Ben Leedle can't sit still. That is not only because he runs American Healthways, the country's fastest-growing small public company. Or even because he has four children to chase around. Rather it happens that Leedle's wife, Liz, is a certified physical trainer. "Trust me. There are very few times when we are just hanging around," says the 43-year-old CEO, who played football at Central College in Pella, Iowa. "It's not really our style." But Leedle's style is hardly unique among fellow members of his baby-boom generation. "The boomers have always been hopeful," says Karen Larson, the 61-year-old CEO of Synovis Life Technologies in St. Paul, No. 12 in this year's FSB 100 ranking. "Now they think they are going to live forever."

And who could possibly have sold the boomers on that outlandish outlook? For starters, about a quarter of the 100 companies that are featured in the pages ahead. Larson admits that an "important component" in the expansion at Synovis, a maker of medical products, is the rise in gastric-bypass surgery, which will "continue to grow at a very important rate." So will fertility treatments. IntegraMed America (No. 13) aids in the manufacture of baby-boomers' babies. Arthritis acting up? Exactech (No. 64) makes hip and knee replacements and is about to elbow its way into the shoulder business. "Demographics are supplying more and more candidates for us," says co-founder Dr. Bill Petty, 61, an orthopedic surgeon who is the CEO of the company, based in Gainesville, Fla.

Topping the FSB 100 for the second year in a row, Nashville-based American Healthways helps health insurers manage patients who suffer from chronic illnesses. "Before, the kind of disease-management we do was thought of as a good idea," says Leedle, a 20-year veteran of the company. "But it's becoming untenable for employers to withstand the rise of health-care costs. So companies now view it as a bona fide strategic imperative." American Healthways offers programs covering about 18 conditions--asthma, diabetes, and heart disease, to name a few--but Leedle sees opportunities in areas such as cancer and obesity. He expects that Medicare, which will start paying for disease-management programs in 2005, will also fuel growth. "In the end, we're trying to save companies money while improving health care," says Leedle.

What he is describing is the mania to boost efficiency, which is finally reaching health care--much as it earlier hit industries such as manufacturing and retailing. "We're seeing the outsourcing of various functions in health care," says Mark Zandi, chief economist at consultant Economy.com in West Chester, Pa. "That's why companies involved in those activities are doing so well."

No surprise, either, that a handful of FSB 100 companies--including Quality Systems (No. 15) and LanVision Systems (No. 33)--are focused on incorporating technology into medical practices. "Technology is key to driving productivity," notes Zandi. And not just in health care. Epiq Systems (No. 36), for example, deploys its software to help sickly companies negotiate their way through bankruptcy.

Two other trends in this year's ranking: First, technology is coming back. Last year's list had just two technology firms, while this year there are ten. Also, the oil industry has suddenly gone flat, with only two firms represented, vs. nine a year ago. That sounds odd considering recent pumped-up oil prices. But those prices weren't reflected in 2003 financial results. With demand accelerating, the FSB 100 should be gushing with oil drilling and related supply businesses next year.

That is not to suggest that every fast-growing company is tethered to an exploding industry. "Our growth is not a function of high demand, it's a function of grabbing market share," says Philip Hawk, CEO of Team (No. 46). "To our customers, we're a necessary evil." The Alvin, Texas, company services pressurized piping systems used in such manufacturing environments as refineries, paper mills, and power plants. "Our industrial plants are getting older," says the 50-year-old Hawk, who answers his own phone. "Over time, the pipes get points of weakness or areas of corrosion in them. And where there's deterioration, our job is to come in and do the rehabilitation. We fix them so they can keep running." For a minute there, it sounded as if he, too, was talking about the baby-boomers.