Routine Phenistix urine testing in antenatal clinics in Derby (5,000 births annually) revealed an undiagnosed case of phenylketonuria in an otherwise healthy young married woman of I.Q. 65. A synthetic diet containing 30 mg/kg/day of phenylalanine kept her blood level between 2 and 6 mg/100 ml. Pregnancy was terminated at 38 weeks, with the birth of a 1,910 gm girl whose sole physical abnormality, apart from low birth weight, was a single umbilical artery. After a transient tyrosinemia she has developed normally; her growth recovered to about the 25th percentile. Her developmental quotient at 10 months of age was 98. It may be as suggested by Auerbach that positive urine tests for phenylketones in the mother are a better guide to risk of fetal brain damage than arbitrary levels of blood phenylalanine, in that this gives a direct indication of body handling of the raised blood phenylalanine. However, we still do not know if the baby has normal intelligence because of, or in spite of, the diet.