(FORTUNE Magazine) – Before you resort to extreme measures like caloric restriction to add a few years to your life, wouldn't it be nice to know how many years you have left?

Sure it would. And one doctor is eager to help. Michael F. Roizen, an internist who now practices in Syracuse, N.Y., has built a mini-empire called RealAge that consists of two New York Times bestsellers (a new book came out this month), a hugely popular website, even a TV show in Europe--all based on a mathematical system that purports to figure out exactly how fast your body is falling apart. You know your chronological age, his thinking goes. But how old you really are--what you might call your biological age--depends on a host of factors: how many cigarettes you've puffed, martinis you've downed, jogs you've taken. By placing a numerical value on 78 behaviors that he maintains can add or subtract years from your life, he has moved aging from the abstract to the concrete. It's one thing to know it's not good to smoke. It's a lot more meaningful to discover that smoking can knock 7.2 years off your life expectancy. Though Roizen's estimates aren't perfect--we all age at slightly different rates--researchers say the numerical values he uses generally fit the findings of independent epidemiological studies.

You can take a free test on RealAge.com to determine how your chronological age compares with what Roizen calculates your actual age to be. But for the most detail, you fly to Chicago and fork over $1,950 for an all-day physical at the University of Chicago Hospital's Program for Executive Health, launched by Roizen in 1999. I book an appointment. A week before I'm due to leave, I catch Roizen on Oprah with two women. Rebecca, a 42-year-old mother of four, never exercises, has high blood pressure, and is 25 pounds over her fighting weight. Roizen calculates her RealAge as 50. Fifty-two-year-old Terry, a smoker who is partial to full-bodied Pepsis, is worse: Her RealAge comes in at 62.

On my flight to Chicago, my wayward youth keeps flashing before my eyes. Like most guys who grew up in a newsroom, I smoked a pack or two back then, and two-martini lunches and I were not exactly strangers. But, hey--I stopped smoking 30 years ago, and I don't drink much anymore. I eat sensibly and take an aspirin and vitamins every day. I live in a world of deadlines, which isn't great for stress, but I play squash regularly and work out three or four times a week. By the time the plane sets down, I'm ready to rumble. Okay, Dr. Roizen, give me your best shot. My chronological age is 68. What's my real age?

I arrive at the hospital promptly at 8 A.M., check in, and am hustled downstairs to have six vials of blood drawn. After a quick breakfast, I'm escorted to a dermatologist who gives me a head-to-toe exam. Then it's upstairs for a visit with Dr. William Harper, medical director of the executive-health program, who will be my primary doctor for the day. He has studied my records and wants to hear about the tear in my rotator cuff, my headaches, exercise patterns, and lots, lots more.

I slip into a gown for a physical that covers all the bases, including some I never knew I had. Next stop is a visit with an audiologist, who agrees with my wife: I have hearing loss in the high tonal ranges. I'm checked for glaucoma (negative). Now it's stress-test time. A technician hooks me up to the EKG and blood-pressure-monitoring equipment, and I'm off and running. I survive with flying colors and am rewarded with a turkey burger and a visit with a dietitian, who says my emphasis on fish, chicken, oatmeal, low-fat milk, and the like is okay, but I should switch to skim rather than 2% milk, substitute olive oil for mayo, and add more tomato products, yogurt, and nuts to my diet.

The orthopedic crowd says my flexibility is good, and my body fat, at 15.4%, is excellent. An exercise physiologist introduces me to stretches and activities designed to improve my cardiofitness. Then it's back to Dr. Harper. My cholesterol came in at a very respectable 137 (the LDL component at 54), which means, he says, I can cut my Lipitor dosage in half.

Next Roizen and I hit a squash court for an hour of good exercise. Then it's out to dinner and more good news. My chronological age may be 68, but my RealAge is 57. That means I'm likely to live to age 85, 11 years longer than the typical guy born in 1935.

I'm relieved and pleased. But Roizen wants more. "You can make many small changes in your diet and lifestyle that will have a big impact on your rate of aging," he tells me. They include eating fiber-rich wheat bread dipped in omega-3-packed olive oil before dinner, having a drink or two a day, increasing my heart rate when I'm on a treadmill or exercise bike, and walking 30 minutes a day. I also must report to him by phone or e-mail once a day for the next two months--a good incentive to stick with the program.

How much more will that additional stuff turn back the clock? About four years, says Roizen. But the really good news, he adds, is not so much that I'll live longer--it's that those added years are likely to be healthy ones.