Sixty-two families with fetal diphenylhydantoin exposure were studied during two or more pregnancies. In 15 of these families at least one of the exposed children had some of the physical effects of DPH exposure ("affected" families); in the remaining 47 families no exposed child was affected ("unaffected" families). Review of 62 family histories and pedigrees was not helpful in differentiating these two groups for counseling purposes. However, mothers who had one affected child appeared to be at much higher risk for having subsequent affected children (9 of 10) if phenytoin use was continued through future pregnancies than were mothers whose first-born child was unaffected despite being exposed to phenytoin during the pregnancy (5 of 52 among all families, or 1 of 48 when only children exposed throughout the entire pregnancy were included). The difference between families with the first exposed child affected and first exposed child unaffected was highly statistically significant (p less than 0.0001). School and learning problems and developmental or mental retardation were present in both groups, and significantly more frequently in affected families. Physical and growth abnormalities were noted in both affected and unaffected family groups, also at a significantly higher rate in affected families.