The objectives of this study were to establish if, and to what extent, benzbromarone affects allopurinol/oxypurinol kinetics, and to compare the uric acid lowering capabilities of Allomaron (allopurinol 100 mg plus benzbromarone 20 mg) with the effects of allopurinol alone in patients with confirmed gout. We studied 14 adult men in an open randomized cross-over study. After a 14 day run-in period with Zyloprim (2 x 100 mg allopurinol tablets in the morning), the patients were randomly allocated to morning doses of either Allomaron (2 tablets) or Zyloprim (2 tablets). Seven days later cross-over was effected and the alternative treatment was taken for a further 7 days. On days 7 and 14 the patients came into hospital and venous blood samples were taken over 24 h for allopurinol and oxypurinol assays by HPLC. Serum uric acid was determined on days -14, 1, 7, and 14. Benzbromarone lowered plasma oxypurinol concentrations (Allomaron/Zyloprim mean ratio of AUC0-->24 was 59%; 95% confidence interval 54-64%), but did not affect plasma allopurinol concentrations. Despite this pharmacokinetic interaction of benzbromarone with allopurinol, resulting in lower plasma concentrations of oxypurinol, Allomaron was superior to allopurinol alone in lowering serum uric acid, probably because of the added uricosuric effect of benzbromarone.