We measured 11 amino acids in untimed urine samples, to determine whether which samples are suited for use in diagnosis of aminoacidurias. Results of untimed samples varied by as much as 25% more than for 24-h collections when amino acid excretions were expressed in terms of urinary creatinine. Values decreased with increasing age for either type of specimen. Urinary amino acid excretions were also determined with untimed or 24-h samples from patients with cystinuria, Lowe's syndrome, nonketotic hyperglycinemia, or phenylketonuria. In all cases studied, the amino acids diagnostic of the diseases significantly exceeded the reference interval obtained for 260 control subjects in six age categories. We conclude that untimed urine samples can be used for diagnosis of these inborn errors of amino acid metabolism, but further studies are needed to evaluate their usefulness for other metabolic disorders.