Two hundred two recurrent calcium oxalate stone-forming patients with idiopathic hypercalciuria or hyperuricosuria, or both, were treated for an average of 2.91 years (1 to 7 years) with thiazide or allopurinol, or both. The frequency of new stone formation was drastically reduced. During the treatment period of 625 patient years, 220.0 new stones should have occurred, whereas 22 were actually formed (chi-square=178, P less than 0.001). Thirty-four patients without discernible metabolic disturbances and treated only with increased fluid intake and dietary advice formed 29 new stones compared to a predicted 33.2 stones (87.3%). Thirty similar patients treated with thiazide and allopurinol formed six stones compared to a predicted 31.8, P less than 0.001. Chronic reversal of idiopathic hypercalciuria and hyperuricosuria with thiazide and allopurinol is an effective way to prevent recurrent calcium oxalate stones. Conservative measures are only of marginal effectiveness in treating metabolically normal stone forming patients; however, thiazide and allopurinol appear to decrease new stone formation.