One million 6-week-old infants were screened for aminoacidurias and the long-term follow-up analyzed to assess the benefits of the screening program. Apart from phenylketonuria, now normally detected by blood screening at five days, the most frequent abnormalities identified were cystinuria, histidinemia, Hartnup disease, and iminoglycinuria. Other disorders occurred less frequently than 1:100,000. Early diagnosis provided unequivocal clinical benefit only for phenylketonuria. There was probable benefit to patients with cystinuria, homocystinuria, argininosuccinic aciduria, and to some patients with Hartnup disease. However, benefit of early diagnosis in these disorders, of which the combined incidence was 1:10,000, was not clear-cut; for example, in 68 cystinuric children, four had already developed renal stones despite close medical supervision and a regimen of increased fluid intake to the limits of tolerance. No patient detected with any other condition benefited, either because the condition appeared benign and was not reated, or because the disorder was serious or lethal and there was a bad outcome despite early diagnosis and treatment. Existing urine screening programs should explore the incidence and clinical significance of further biochemical abnormalities detectable in the newborn infant, but there is no indication at present for the initiation of new urine screening programs designed to detect only aminoacidurias.