A spectrophotometric method for the determination of cysteine and cystine in urine is described. This method is a modification of that reported for the determination of cysteine and cystine in proteins. The determination in urine is specific and simple. The colour develops at room temperature and no pre-treatment of urine is needed. Other amino acids, urea, uric acid, ascorbic acid, bilirubin, biliverdin, carbohydrates, hormones, acetone, salicylic acid, small amounts of protein and other common components of urine do not interfere. The method is particularly suitable for the diagnosis of hepatic cystinuria and other diseases characterised by high sulphur-containing amino acids in urine.