The simplest way to diagnose phenylketonuria, a ferric chloride test on urine samples, is still of great usefulness even in the newborn period when babies are most available for testing. Twenty-two of our 60 patients were less than 1 year old at the time of the diagnosis, but ten infants in particular who were less than 60 days of age also had "typical" biochemical findings. The proper collection of urine samples to avoid bacterial contamination resulted in a positive ferric chloride test in seven of these 10 infants who had been examined by 3 weeks of age. Several infants tested earlier were positive on the second, fifth, and eighth days of life. This method then may be employed to make an early diagnosis or to confirm a suspected diagnosis when only an elevated blood phenylalanine is available.