Faith in medicine runs deep in America. We spend more per person on health care than any other nation. Most of us are confident that we will live longer, more active lives than our parents. Whether we eat too much or exercise too little, whether we're turning gray or feeling blue, we increasingly look to some pill or procedure to make us better. No one likes to hear official projections such as those that came out last week about Medicare, which show that the program will be running multitrillion-dollar annual deficits just when baby boomers need it. But a common response is: What's a more important priority for society's resources?

Good question, assuming that devoting ever more dollars to medicine will bring us longer, healthier lives. But there is mounting evidence that each new dollar we devote to the current health care system brings small and diminishing returns to public health. Today the United States spends more than $4,500 per person per year on health care. Costa Rica spends less than $300, and has half as many doctors per capita. Yet life expectancy at birth is nearly identical in both countries.

Despite the ballyhooed "longevity revolution," life expectancy among the elderly in the United States is hardly improving. Since 1990 Medicare expenditures per senior have more than doubled. Yet life expectancy among American women at age 65 was lower in 2003 than it was in 1991, according to estimates released by the Social Security Administration last week. Yes, we are an aging society, but primarily because of falling birthrates.

Younger Americans, meanwhile, are far more likely to be disabled than they were 20 years ago. Most affected are people in their thirties, whose disability rates increased by nearly 130 percent, due primarily to obesity. Americans of all ages are also increasingly likely to die from a host of infectious diseases and chronic conditions. Between 1980 and 2000, the age-adjusted death rate from diabetes increased by 39 percent, chronic lung disease by 49 percent, and kidney disease by 21 percent.

Why has our huge investment in health care left us so unhealthy? Partly it is because so many promised "miracle cures," from Interferon to gene therapies, have proven to be ineffective or even dangerous. Partly it's because health care dollars are so concentrated on the terminally ill and the very old that even when medical interventions "work," the gains to average life expectancy are small. And partly it is because of medical errors and adverse reaction to prescription drugs, which cause more deaths than motor vehicle accidents, breast cancer or AIDS. Each year roughly 200,000 seniors suffer fatal or life-threatening "adverse drug events" due to improper drug use or drug interaction. Will Medicare's new prescription drug benefit save more lives than it ends? The answer is not obvious.

There are some simple ways to improve the effectiveness of medicine. Each year 90,000 patients in the United States die from infections they contract in hospitals, and doctors and nurses who fail to wash their hands are the biggest vector. To cut down on medical errors, many hospitals are adopting sophisticated quality control measures similar to those used by manufacturers to reduce "defect rates." Today only 1 cent out of every dollar spent on the National Institutes of Health goes to establishing "best practices" in medicine. Redirecting more funds from basic research to studying the effectiveness of different treatments would go a long way toward preventing such lethal medical fads as high-dose chemotherapy and bone marrow transplants.

But it is also time for Americans to recognize the limits of medicine. Why don't Americans live any longer than Costa Ricans? Overwhelmingly, it's because of differences in behavior. Americans exercise less, eat more, drive more, smoke more, and lead more socially isolated lives. Even at its best, modern medicine can do little to promote productive aging, because by the time most people come in contact with it their bodies are already compromised by stress, indulgent habits, environmental dangers and injuries. Today, about 127 million adults in the United States are obese or overweight, 47 million still smoke, 14 million abuse alcohol, and 16 million use addictive drugs.

Yes, there is a better way for government to spend its money than on simply expanding Medicare or covering its yawning deficits. For example, it could offer people who avoid large lifestyle risks partial relief from Medicare taxes and premiums. It could regulate sprawl to make walking possible for more Americans. And it could put far more effort into discovering and providing more effective treatments for addiction. Unless we adopt such approaches, life expectancy for the next generation of seniors is likely to decline, even as the cost of treating chronic disease and disability overwhelms the next generation of taxpayers.

Phillip Longman, a senior fellow at the New America Foundation, is the author of "The Empty Cradle: How Falling Birthrates Threaten Global Prosperity and What to Do About It."