The number of Americans more than 100 years old has soared by almost two-thirds since 1980 and will probably increase even more substantially by the year 2000, according to a new Census Bureau report.

The country's population of centenarians has reached 25,000, compared with about 15,000 recorded in the 1980 census, the Census Bureau said. That makes the oldest segment of the U.S. population the fastest-growing.

If current mortality trends continue, in 2000 the nation will have about 100,000 people over age 100, the census said -- still a relatively small group but far less rare than only a few years ago.

The study said the growth in centenarians reflects improvements in medical care and living conditions, which have raised life expectancy, as well the substantial increase in immigration and births in the United States during the 1880s.

"The country was growing like crazy 100 years ago," said Gregory Spencer, a Census Bureau demographer who was senior author of the new report. "With the improvements in longevity, we're getting very dramatic growth now."

The report, based on Census Bureau counts for 1980 and Social Security Administration records for 1986, was prepared under contract for the National Institute on Aging as part of its special focus on centenarians, marking the 100th anniversary of the National Institutes of Health.

"We haven't studied this group very much in the past," said Richard Suzman, a health science administrator at the National Institute on Aging. "We're beginning to now as many more people survive to this age."

According to the most recent Social Security data, almost 80 percent of the centenarians are women, an even greater imbalance of the sexes than among the 2.6 million persons over age 85, about 70 percent of whom are women.

However, according to the 1980 census tabulations, which have been revised for the new report, a much higher proportion of the 100-year plus women live in nursing homes than do men -- more than half compared with less than one-third.

"Women's life expectancy has gone up faster than that of men," Suzman said, "but the women are more likely to get disabling diseases that are not fatal, such as arthritis."

Another factor keeping more of the men out of nursing homes is that more are still married when they reach 100, rather than widowed.

"Most of women are left without their spouses," said Spencer. "In our culture, for whatever reasons, older men can marry down to a younger age group but women can't."

Suzman said many of the centenarians not only outlive their spouses but also their assets, and become dependent on government aid. "An overwhelming issue becomes the need for long-term care," Suzman said. "That's extremely expensive."

Although the 1986 estimate has no data by race, the figures for 1980 indicate that a slightly higher proportion of blacks reach their 100th birthday than whites, a reversal of the pattern for other age groups.

Spencer suggested that blacks who reach 90 are a more select and possibly healthier group than their white counterparts, hence they are more likely to survive to their 100th birthday. However, he noted, some experts believe this "crossover" in mortality trends may be the result of poorer data on the actual age of elderly blacks.

According to the Social Security data, the number of centenarians in Virginia rose from 302 to 411 between 1980 and 1986, and from 268 to 333 in Maryland during the same period. The number fell from 92 to 74 in the District of Columbia. Jacob Shmulowitz, director of statistics analysis for Social Security, said the drop may reflect some D.C. centenarians' going to nursing homes in the suburbs.