Contrary to the usual statements that cystinuria is a medical curiosity and a benign disease, observations of 16 individuals, including seven children, have shown that the consequences of cystine stone formation are of such serious nature as to markedly shorten life expectancy and induce both physical and mental anguish in involved persons. Thus, therapy directed toward the removal of the offending urinary cystine is warranted. This study shows the unequivocal value of penicillamine administered orally to patients with cystinuria; penicillamine will lower urinary cystine to levels which have not allowed further stone development in the treated individuals during the trial observation period. Further, early side effects have been surmounted, and no late effects have been noted up to one year of treatment. Suggested dosage of penicillamine is 30 mg/kg of body weight per day.