Two cases of maternal phenylketonuria are presented. The first woman, untreated, had 3 children. Her first child was of normal intelligence but had serious personality problems. The second had a major cardiac abnormality incompatible with life. The third had phenylketonuria and was successfully treated from early infancy. The second woman was treated on a low phenylalanine diet from 8 weeks gestation and gave birth to a child without congenital abnormalities who is now 23 months old and is of normal intelligence.