Ninety-five infants from 26 to 42 weeks of gestational age were studied with a simple standardized cerebral transillumination technique utilizing a Chun gun fitted with a clear flexible disc to establish normal measurements and to test the hypothesis that transillumination is useful in differentiating "catch-up" growth from hydrocephalus. One group of infants were measured at birth; the other group were infants grown to similar gestational age and serially measured. Data from these groups were found to be similar. Transillumination was not found to be a useful technique for separating "catch-up" growth infants from the normal population, but was found to be potentially useful in separating the normal and "catch-up" population from children with hydrocephalus or serious anatomic abnormalities of the central nervous system.