Further improvements in fighting killer diseases such as cancer and heart disease could have bad as well as good effects on the country, a new report warns.

In addition to the obvious benefits to those would have died prematurely from such diseases, a longer lifespan for large numbers of the population could have an un- healthy, long-term impact on the federal budget and "achieve the dubious goal of keeping people alive well beyond their years of vigor," says the Population Reference Bureau.

The private, nonprofit group released a new report saying that "remarkable" medical advances "mean we can all look forward to longer lives," but "their impact on society and on the current system of public programs for the elderly could be disastrous."

The report urged policy makers to start making needed adjustments in federal planning. "The postponement of death increases federal costs, requiring more taxes. If we anticipate the timing and magnitude of these cost increases, we can meet them or adjust programs to minimize the economic burden."

It also recommended that federal research dollars should be better balanced between studies designed to find ways of slowing the aging process and those designed to fight specific diseases. "While death is delayed, aging is not," said the report, which noted that research dollars directed toward heart disease and cancer outnumber those for aging research in general by 20 to 1.

The population group's report even suggests that more money be spent on medical research that would help men catch up with women's longer average life span. "Focusing research on diseases which disproportionately kill men could allow men to live longer and continue to provide for their wives, as well as receive a better return on their Social Security investment," it says.

The report, titled "Death and Taxes: The Public Policy Impact of Living Longer," draws on the research of seven scientists, including economists, demographers and a biologist.

It estimates that federal expenditures on individuals 65 years and older, currently about one-fourth of the annual budget, may grow to about 50 percent of the budget by the year 2025.

In 1983, U.S. life expectancy at birth stood at 74.5 years for all Americans, 70.8 for men and 82.2 years for women. By the year 2000, many experts believe average life expectancy could be as high as 80 years, 74 years for men and 86 for women.

Much of the recent improvement in life expectancy has been due to a drop in premature deaths due to heart disease. While those who might have died continue to pay more taxes, they ultimately also claim more in benefits, the report notes.

And if, for example, scientists had succeeded in eliminating early deaths from heart disease in 1978 alone, it might have cost the government about $15 million extra in old-age benefits, according to estimates by government economists Douglas Norwood and Barbara Boyle Torrey.

The report comes in the wake of increasing national debate about the impact of an aging population.

Rather than questioning whether we can afford to care for the elderly, "the challenge is to plan efficiently for an aged population," says economist Torrey, one of those cited in the report. "Any society that plans, especially a society as diverse and wealthy as ours, should be able to cover these kinds of costs," she said yesterday. Torrey, currently with the Census Bureau, said she was speaking personally and not voicing administration policy.