University of Chicago researchers conclude in a new study that even curing leading killers such as cancer, heart disease and diabetes would add no more than a handful of years to the average American's life.

The study takes direct aim at the notion that medical science can continue to significantly extend life expectancy, and suggests that the United States should dramatically shift its priorities from prolonging life to improving the quality of life.

The theoretical upper limit of average life expectancy, according to the researchers, is about 85 years. In the United States, average life expectancy now is 79 years for women and 74 for men.

Even if all cancers were miraculously cured tomorrow, the average American's lifespan would increase by only three years, according to the new analysis of death rates, which appears in today's issue of the journal Science.

"Once you go beyond the age of 85, people die from multiple-organ failure. They stop breathing. Basically, they die of old age. And there's no cure for that," said S. Jay Olshansky of University of Chicago and Argonne National Laboratory, the study's principal author. "Barring a reversal of human aging on a molecular level, the rapid increases in life expectancy are over."

"We've accomplished the first step. We're living longer. Now we should concentrate on diseases that give aging a bad name," said Jacob Brody, dean of the School of Public Health at the University of Illinois in Chicago.

"Congress likes to spend money to live longer, but we're already living longer. Congress, and almost everybody else, is in love with longevity -- mindlessly in love," he said. "My God, we're already living long enough. Let's make life worth living for the aged."

"Personally, I think we'd improve the quality of life much more for the aged if we found a cure for Alzheimer's and arthritis, rather than cancer," added Olshansky.

Olshansky and his colleagues, Christine Cassel of Chicago and Bruce A. Carnes of Argonne, warn that further gains in longevity will lead to a dramatic rise in the aged population. While Americans might live longer, the researchers question whether they will live better. Frailty, and disabling but not life-threatening illnesses of old age, could condemn a growing population to aching joints, blindness, deafness, incontinence and dementia.

Olshansky and his colleagues calculated that even if major diseases were eliminated, average life expectancy would increase little. If all cancer and all heart disease were eliminated, the average life expectancy would grow by about three years. Even eliminating all deaths from cancer, diabetes and circulatory disorders would only increase average life expectancy 15 years, but most people would probably not realize that gain because their bodies would succumb to normal aging.

Indeed, death rates for children and young adults in the United States are so low that eliminating all causes of death before the age of 50 would only increase overall average life expectancy by 3.5 years.

Thanks largely to reductions in infant and maternal mortality, Americans have experienced a dramatic increase in average life expectancy since the middle of the last century -- from 40 years to near 80 years today. But each gain in life expectancy is increasingly difficult.

Many biologists have considered 85 years to be the upper limit for average life expectancy, because normal aging processes tend to wear out cells and organs at about that age. The study's authors caution that this theoretical limit refers only to average life expectancy. There will always be vigorous individuals who live far past their 85th birthday, just as there will always be people who die young.

The limits on lifespan proposed by Olshansky and colleagues were challenged yesterday by rival researchers who offer a rosier vision of the future.

By reducing major risk factors -- in other words, reducing blood pressure, obesity and smoking, for example -- some demographers believe average life expectancy could reach 98 or 99 years.

"That would be hard to achieve, but not impossible," said Kenneth Manton, a demographer of the aged at Duke University, who points to some populations -- such as Mormon high priests -- who have a life expectancy exceeding 85 years. "I think that we don't yet know what the limits of life expectancy are."