The frequency of severe reactions to allopurinol has probably been underestimated. A retrospective study encompassing a five-year period has yielded 20 patients with severe hypersensitivity reactions to allopurinol. Patients with preexisting renal impairment or who were receiving concomitant thiazide diuretics appeared to be especially predisposed. Cutaneous reaction patterns included maculopapular eruptions, exfoliative dermatitis, and toxic epidermal necrolysis. eosinophilia was uncommon. Forty percent of the patients developed hepatic involvement and 45% had renal involvement. Hepatic and renal changes usually were reversible and were not unique to any one cutaneous reaction pattern. Three patients with renal involvement required prolonged administration of systemic steroids. Complications included sepsis, decubitus ulcers, and thromboembolism. Two patients required hyperalimentation. Sequelae included dry eyes, pigmentary disturbances, and keloids. Three patients died as a result of their reaction. It is concluded that allopurinol should be used only in select patients, and the dosage should be modified if renal disease exists.