Both urinary calcium excretion and renal stone episodes are increased during the summer in Finland. Since thiazide and unprocessed bran are known to decrease urinary calcium excretion, we treated 73 patients with recurrent urinary stone formation by giving them unprocessed bran and intermittent thiazide. Of the patients, 32 had absorptive hypercalciuria and 41 had normal urinary calcium values. All patients were on a low-calcium and low-oxalate diet and took 40 g bran daily. Fourteen of the hypercalciuric and 14 of the normocalciuric patients were randomly allocated to use hydrochlorothiazide 50 mg b.i.d. from May to September. Reduction of stone formation was seen in all groups. The combination of thiazide + bran was superior to the bran on its own in inhibition of stone formation. Only 3/11 (27%) stones passed through during the summer in the thiazide + bran group as compared with the 11/17 (65%) in the bran group.