The plasma concentrations of oxipurinol, the chief metabolite of allopurinol, were studied in 66 patients with gout in whom the dose of allopurinol varied between 100 and 400 mg per day. Renal function ranged from normal to moderately impaired. Plasma oxipurinol concentrations correlated directly with both allopurinol dosage and with renal glomerular function as reflected by the plasma creatinine concentration. Plasma oxipurinol concentrations between 30 and 100 mumol/l were generally effective in controlling hyperuricaemia. However, oxipurinol concentrations usually rose above this range if the daily dose of allopurinol exceeded 300 mg in patients with plasma creatinine concentrations of 0.2 mmol/l or more. In patients with normal renal function, a rise of the plasma xanthine concentration to between 6 and 9 mumol/l suggested a satisfactory degree of xanthine oxidase inhibition. These measurements are particularly useful in patients who are still hyperuricaemic despite the usual doses of allopurinol.