By a retrospective study on a two-year period (1494 patients) we have obtained evidence that the best method of screening aminoacidopathies is given by the ion-exchange chromatography of amino acids. This method gave the highest rates of diagnosis and allowed diagnosis of aminoacidopathies that other techniques such as thin-layer chromatography would fail to detect because the concentrations of some amino acids, although pathological, were too low. The diagnostic rate was markedly improved by a skilled clinical selection of the population studied.