This report describes biomedical causes of mental retardation (MR) among school-age children and associated medical conditions in children for whom no cause was reported. This study involved 715, 10-year-old children with MR (intelligence quotient [IQ] 70 or less) born between 1975 and 1977. We determined biomedical causes of MR using a hierarchical approach based on the timing of the event (i.e. prenatal, perinatal, or postneonatal). Among children with no identified biomedical cause the occurrence of associated medical conditions was examined. No reported biomedical cause could be found in 78% of children with MR (87% mild, IQ 50 to 70; 57% severe, IQ < 50). Prenatal causes were present in 12%, perinatal causes in 6%, and postneonatal causes in 4%. On the basis of these findings it was concluded that intensive use of public health prevention strategies can reduce the number of children who receive a diagnosis of MR.