Deadly brain tumors are striking elderly Americans at an alarmingly increasing rate, government researchers reported.

While at a loss to explain the upsurge, researchers at the National Institutes of Health found the occurrence of malignant brain tumors had increased "dramatically" between 1973 and 1985 among the elderly in the United States.The greatest increase occurred among those age 85 and older, with the incidence apparently jumping fivefold in that group, the researchers said Tuesday. But the incidence also nearly doubled among those 75 to 79 and nearly quadrupled for those ages 80 to 84, they said.

While the overall cancer rate increased by only 0.9 percent each year during the same period, the increase in brain cancers translates into a jump of more than 23 percent a year for those 85 and older.

Moreover, the most common types of brain tumor were those known as glioblastoma multiforme and astrocytoma, which are "highly malignant and invariably fatal," the researchers said.

Despite the increase, the researchers noted malignant brain tumors remain relatively uncommon. There are only about 15,000 new cases in the United States each year, representing only about 1.5 percent of all cancer cases.

Nevertheless, the apparent increase is "alarming," said Nigel Greig, a neuroscientist at the National Institute on Aging who led the new study, published in the Journal of the National Cancer Institute.

The findings are based on data collected in Connecticut, Iowa, New Mexico, Utah, Hawaii, Atlanta, Detroit, San Francisco, and Seattle as part of the National Cancer Institute's Surveillance, Epidemiology and End Results, or SEER, program, which gathers cancer data from a representative sample of 10 percent of the U.S. population.

Greig indicated the increase could have been caused by something in the environment. But too little is known about brain tumors to even speculate, he said, and much more research is needed to explore the issue.