The fetal hydantoin syndrome is a variable pattern of altered growth and performance which includes unusual facies, distal phalangeal hyoplasia, and other defects occurring in some infants exposed in utero to hydantoins. A prospective study of 35 infants exposed prenatally to this class of anticonvulsants showed that 11% had sufficient features to be classified as having the fetal hydantoin syndrome. An additional 31% displayed some features compatible with the prenatal effects of hydantoins. A case-control study of 104 infants whose mothers received hydantoins during pregnancy supports these conclusions. Reduction of intellectual ability in infants with the fetal hydantoin syndrome is the area of greatest concern. Women being treated with hydantoin anticonvulsants should be told of the nature and magnitude of risks to the developing fetus before considering a pregnancy.