Hypersensitivity reactions to allopurinol, a drug commonly used in the treatment of hyperuricemia, are being reported with increasing frequency. Of thirty-eight patients reviewed herein (including seven from our hospital and thirty-one from a review of the literature), ten deaths (26%) were related to complications of allopurinol hypersensitivity. Preexisting renal disease was present in 97% of patients, and, in the majority of these, the dosage of allopurinol was not reduced despite instructions contained in the package insert for this drug. At least 78% of patients were taking a thiazide diuretic prior to starting allopurinol therapy. Over 60% of patients had received allopurinol for asymptomatic hyperuricemia. Hallmarks of this hypersensitivity syndrome include a prolonged illness initially manifested by fever, a prominent cutaneous reaction, eosinophilia, hepatic abnormalities, and acute renal failure. Other involvement such as gastrointestinal bleeding is common. The mechanism of the hypersensitivity reaction is not clear, but it may represent an immune complex disease prolonged by the persistence of a currently undefined antigen. Treatment with systemic corticosteroids, often for several months, is usually necessary for the gradual resolution of this potentially fatal syndrome.