Allopurinol and thiazide effects on new urinary stone formed after discontinued therapy in patients with urinary stones. We treated 87 patients with calcium-containing urinary stones with either allopurinol alone (44 patients) or in combination with thiazide (43 patients) and studied new stone formation before, during, and after the discontinuation of the drug therapy. The number of stones formed were 1.18, 0.24, and 0.13 before, during, and after discontinuation of the drug therapy, respectively, in the patients treated with allopurinol alone and 1.32, 0.20, and 0.09 in those treated in combination with thiazide. No differences were observed in these values and the duration of each observation period between the two groups. Decreases in the incidence of stone formation even after interruption of drug therapy suggested that recurrence-preventive effects observed following administration of these drugs include the effects of medical guidance. However, allopurinol therapy was effective in preventing recurrence in patients with hyperuricosuria. 