The family histories of ^6 developmentally disabled patients are examined. Three hypotheses concerning the correlation between diabetes mellitus and congenital anomalies are tested. The authors feel that this correlation must be due to some factor in either the fetal genotype, the maternal genotype or the environment of the mother. The ages of all persons in the family histories are estimated (actual ages were not available). Four groups taken from the family histories are compared to the United States population. These are the mothers (381), fathers (378), paternal relatives of patients (2,352) and finally the maternal relatives (l,99l) of patients whose mothers were not diabetic. It was found that only the group containing the mothers of patients differed significantly from the control population with a higher prevalence of diabetes. The authors drew the conclusion that some factor in the environment of the diabetic mother causes the correlation between diabetes and congenital anomalies. The authors postulate that insulin might be this environmental factor. Diabetes and its effect upon pregnancy is discussed. Literature on diabetes and congenital anomalies is reviewed as are studies concerning possible environmental teratogenic agents.