In this study of 17 renal transplant recipients with hyperuricemia the effects of allopurinol vs. benzbromarone were compared. Both drugs effectively lowered serum uric acid concentrations by 19 vs. 35% of pretreatment values. Adverse reaction to allopurinol consisted in augmenting of azathioprin toxicity for bone marrow, with occurrence of an isolated fall in the hematocrit in several patients. With benzbromarone no drug interactions were observed. However, one patient exhibited uric acid stone formation in the damaged kidney.