The syndrome of hyperuricosuric calcium oxalate nephrolithiasis, consisting of calcium oxalate stones, hyperuricosuria and the absence of known causes of calcium stones, follows a course of particularly severe stone disease. This course can be dramatically altered by the reduction in new stone formation by the chronic administration of allopurinol. The mechanisms proposed in linking hyperuricosuria to calcium oxalate stones are both dependent upon the presence of oversaturation of the urine with one or both species of uric acid. Allopurinol reduces the saturation of the urine with respect to sodium hydrogen urate and reduces urine concentration of undissociated uric acid, thus directly altering the pathogenetic mechanisms proposed for this disorder.