Cystinuria is an inherited condition affecting the active transport of the diamino acids cystine, ornithine, lysine and arginine across the renal tubule and the small intestine. The only clinical effect is the production of urinary tract stones and if these can be prevented the affected individuals can lead a normal life. In many people cystine stones can be dissolved and new stone formation prevented by a high fluid intake, but if this does not succeed regular treatment with penicillamine will do so. Although many side-effects have been described with penicillamine treatment it is rare for them to be severe enough to prevent its use in patients with cystinuria. Since the clinical effects of cystinuria can be prevented by either a high fluid intake or by penicillamine it is important to make the diagnosis in affected individuals as soon as possible and cystinuria should therefore be considered in all people (regardless of age) who form urinary stones.