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Intellectual performance was investigated in a group of 148 children of epileptic mothers enrolled in a prospective study during pregnancy, and in 105 control children; 121 of the 148 children (82%) were examined together with the control group at the age of 5 1/2 years. The intelligence of 116 study and 104 control children was assessed by both verbal (Wechsler Preschool and Primary Scale of Intelligence) and nonverbal (Leiter International Performance Scale) methods. The prevalence of mental subnormality among the remaining 32 study children was calculated on the basis of psychologic assessments done outside our study, or school achievement, or both. Of the 148 study group children, 131 had been exposed in utero to antiepileptic drugs, most commonly phenytoin (103 exposed). The prevalence of mental deficiency in the study group (1.4%) was either the same or only slightly elevated in comparison with that in the general population. In the control group, there were no mentally deficient children. The mean intelligence quotients obtained at the 5 1/2-year examination were significantly lower in the study group than in the control group. There was no increased risk of low intelligence attributable to fetal exposure either to antiepileptic drugs below toxic levels or to brief maternal convulsions. In a few cases, there seemed to be a genetic connection between epilepsy in the mother and poor intellectual performance in the child. A high number of minor anomalies was associated with a lower mean intelligence quotient in both the study and the control groups. However, those features previously reported as typical of children of epileptic mothers, and those shown to be associated with phenytoin exposure in a previous phase of this study, did not predict low intelligence in the affected children.