Twenty phenytoin exposed children between 48 and 99 months of age had an evaluation of behavior and intelligence by a single examiner who was unaware of exposure status. The controls were 98 children identified at birth as having three or more minor anomalies. None of the children evaluated were mentally retarded. In both, a case-by-case comparison and a comparison of the two entire groups, the phenytoin-exposed children had significantly lower scores for both Performance IQ (PIQ), Full Scale IQ (FSIQ), and Visual Motor Integration Test (VMIT). Similar abnormalities have been found in studies of animals exposed to phenytoin in utero. These results suggest that the teratogenic effects of phenytoin may include an effect on cognitive function.