The long-term efficacy and safety of benzbromarone were studied in 21 nontophaceous hyperuricemic men. Daily doses of 40 or 80 mg of micronized benzbromarone caused a rapid fall in plasma urate which was maintained throughout a study period that lasted up to 1 year. A concomitant marked increase in the clearance of urate indicated that benzbromarone is a potent uricosuric drug. Initial treatment was accompanied by transient hyperuricosuria, following which urinary uric acid reverted toward, but failed to reach completely, pretreatment values. Benzbromarone did not inhibit xanthine oxidase nor did it influence the activity of purine phosphoribosyl transferases. These observations suggest that benzbromarone has no direct effect upon purine metabolic pathways, but exerts its hypouricemic action solely by blocking tubular reabsorption of uric acid. Concomitant administration of aspirin interfered only slightly with the uricosuric properties of benzbromarone. No side effects directly attributable to the drug were observed.