CHICAGO, AUG. 30 -- Three of five people entering nursing homes are put there not for medical reasons but because their families no longer have the energy or resources to care for them, a government study concluded today.

The primary medical reasons given for institutionalization were also more of a disruptive nature -- such as Alzheimer's disease -- rather than an illness requiring sophisticated medical attention, according to a survey by the National Center for Health Statistics.

But the researchers who compiled the government data said they thought the children of older people should not be accused of abandoning their parents, since many of the care-givers are elderly themselves.

"A very substantial proportion of people entering nursing homes now are over 85 years old, and their so-called children are grandparents and sometimes great grandparents," said Ethel Shanas, a sociologist and professor emeritus from the University of Illinois at Chicago.

About 1.5 million Americans live in nursing and personal care homes, with 1.3 million being aged 65 and older. To determine why, the center for health statistics conducted a study in 1985.

Shanas and the center's deputy director, Joan Van Nostrand, analyzed responses from 8,522 next-of-kin and found that in only two of five cases was the main reason for institutionalization medical.

"A third of the caretakers were just burned out," Shanas said. "And the rest either said the patient required more care than they could give or afford or that there was no one at home."