The effects of a fixed combination of 100 mg allopurinol and 20 mg benzbromarone (Acifugan) on plasma and urinary uric acid were compared with those of 300 mg allopurinol alone during standardized dietary conditions (purine-free, isoenergetic formula diet with addition of 2 g ribonucleic acid per day). In addition, the components of the fixed combination were given separately. Diet and ribonucleic acid were taken by healthy volunteers for periods of 14 days, drugs were added from day 8 to day 14. Steady state uric acid values before and during drug ingestion were compared intraindividually to calculate drug effects. The fall of plasma uric acid was not significantly different during treatment with 300 mg allopurinol as compared with combined low dose treatment. Renal uric acid excretion was decreased by 56 and 23%, respectively. Comparing combined low dose treatment with the cumulative effects of its constituents it was found that combined treatment was less effective. With combined low dose treatment there is no need to observe prophylactic measures concerning complications at the site of the urinary tract. Compared with monotherapy with 300 mg allopurinol there is no advantage. However, the frequencies of side effects, which are not dose-dependent, will be cumulative with combined treatment, which is probably a major disadvantage. Combined treatment should not be used therefore with the exception of proven inefficiency of monotherapy.