A life-threatening toxicity syndrome consisting of an erythematous, desquamative skin rash, fever, hepatitis, eosinophilia, and worsening renal function in 78 patients receiving allopurinol is described. In a majority of cases, the development of this syndrome was associated with the use of standard (200 to 400 mg per day) doses of allopurinol in patients with renal insufficiency. In pharmacologic studies, it was demonstrated that the renal clearance of the major metabolite of allopurinol, oxipurinol, is directly proportional to the renal clearance of creatinine (oxipurinol clearance = 0.22 X creatinine clearance -2.87). An inverse linear relation was noted between the serum oxipurinol half-life and the renal creatinine clearance [( serum oxipurinol half-life in hours]-1 = 0.00034 X creatinine clearance in milliliters per minute + 0.0045). Long-term use of 300 mg per day of allopurinol was found to result in elevated steady-state serum oxipurinol concentrations in patients with renal insufficiency (serum oxipurinol concentration in micromoles per liter = -2.5 X creatinine clearance in milliliters per minute + 326). Avoidance of allopurinol or use of reduced doses in patients with renal insufficiency according to proposed guidelines should be adequate to inhibit uric acid production in most patients and may reduce the incidence of life-threatening allopurinol toxicity.