Monitoring of plasma oxypurinol has been proposed to prevent allopurinol side effects. An 89-year-old man developed a severe desquamative rash, fever, eosinophilia, hepatocellular injury and renal failure after allopurinol administration. Eight hours after the last dose, plasma allopurinol was undetectable and plasma oxypurinol was 50 mumol/l. This is the first case in which severe allopurinol hypersensitivity occurred despite a simultaneous plasma oxypurinol concentration within recommended levels (below 100 mumol/l).