In a systematic examination of children from incestuous unions by screening tests for congenital disorders of amino acid metabolism, we found an infant with raised urinary excretion of cystathionine, cystine derivatives and isulphides and serious psychomotor retardation. The serum cystathionine and disulphide level was found to be elevated. A tolerance test with methionine and vitamin B12 and tests of other metabolites in the blood and urine confirmed the diagnosis of primary cystathioninuria, the first recorded case in the Czechoslovak population. A differential diagnosis against all the other known congenital metabolic disorders involving the sulphurated amino acids was carried out.