THE ASSOCIATION between low birthweight and mental retardation has been studied repeatedly. The earlier literature on this subject was reviewed by Benton1 in 1940; studies done since then were reviewed in 1962 by Weiner.2 Nearly all the previous investigations were prospective studies of the intellectual abilities of ex-prematures. Most of these reports indicated that children of low birthweight are more often mentally retarded than children of normal birthweight. Harper et al,3,4 Drillien,5 and others6-9 showed prospectively that children of very low birthweight (1,588 gm [3.5 lb] or less) are less often intellectually normal and more often severely retarded than children of normal birthweight, and that the prognosis for children weighing 1,588 gm (3.5 lb) to 2,495 gm (5.5 lb) at birth is intermediate between the prognoses for the very small infants and those of normal birthweight. Therefore, we predicted that a population of severely retarded