Allopurinol is used frequently to treat patients with gout and hyperuricemia. However, adverse effects associated with this agent have been reported occasionally, especially among patients with hyperuricemia complicated with renal diseases. A rise in the blood concentration of oxipurinol, the chief active metabolite of allopurinol, has been noted in patients with renal dysfunctions, pointing to an implication of oxipurinol toxicity. It has been reported that monitoring the serum oxipurinol concentration to maintain in level below 15.2 micrograms/ml (= 100 mumol/l: recommended level) is helpful in avoiding toxicity. At Jikei University Hospital, a survey was conducted on 148 hyperuricemic patients who had been treated with allopurinol at the dosages of 50, 100, 200 and 300 mg daily or 100 mg on alternate days for more than one month. Because oxipurinol is an uricosuric substance, the steady-state serum oxipurinol concentration was determined by HPLC; and creatinine clearance (CCr) was calculated for each patient. 1. In the group composed of patients with normal kidney function (CCr > or = 80 ml/min), increase in the dosage of allopurinol was associated with a linear increase in the serum concentration of oxipurinol. 2. Among the patients with varying renal function who were receiving 100 mg of allopurinol daily, the oxipurinol level increased logarithmically as the creatinine clearance decreased. In some of the patients with renal insufficiency (CCr < 30 ml/min), daily administration of 100 mg of allopurinol resulted in a serum concentration of oxipurinol over 15.2 micrograms/ml. 3. For patients with renal insufficiency (CCr < 30 ml/min), administration of allopurinol at the dosage of 50 mg/day is considered adequate to avoid the accumulation of serum oxipurinol.