RESEARCHERS examining recent government health survey data have observed that the proportion of children reported to have a chronic physical or mental handicap has doubled over the last 25 years. What accounts for it?

The short answer is that scientists don't yet know. Researchers at the University of California at San Francisco, who recently reported this finding, are only in the first stages of a two-year study that will try to determine how much of the observed increase is "real" and how much simply results from more parental awareness of childhood disabilities.

The raw finding is that the number of children under age 18 with chronic impairments has almost doubled to more than 2 million. The increase occurred during a period in which the total number of children remained roughly constant and the number living in dire poverty declined substantially. Medical advances also wiped out such crippling childhood diseases as polio, and surgery provided permanent corrections for certain congenital deformities.

The fact that no comparable increase in disabilities was reported among adults discounts the possibility that new survey techniques account for the reported increase for children. On the other hand, the researchers found no large increase in the percentage of children with very severe disabilities, and they note that increases in reported disabilities have leveled off in the last few years. This suggests that a large part of the reported increase may be attributable to the fact that as more children--especially low-income children--gained access to medical care, their parents became more sensitive to their children's physical and mental health.

The researchers will be trying to sort out the contribution of these and other possible factors--such as parental education, the mother's age at the time of the child's birth, prenatal exposure to toxic substances and the possibility of genetically transmitted defects. Because the researchers are working with a point-in- time survey, however, they are not able to assess the impact of the increased survival rate of premature babies, a factor that other researchers have suggested might contribute somewhat to the incidence of long- term disabilities. Nor will they be able to determine whether childhood handicaps persist into maturity.

Not enough is known to judge whether the society must prepare for continuing increases in the number of handicapped children. But the study has already performed the useful function of reminding people that a large number of handicapped children live in their midst and that, as standards of medical treatment and educational access continue to rise, societal contributions to their care must necessarily increase.