BACKGROUND Immune hemolytic anemia can be caused by sensitivity to many different drugs. In some instances, the sensitizing compound can be identified by in vitro testing, but results are often negative. One reason for this is that a drug metabolite formed in vivo can be the sensitizing agent, but the responsible metabolites have rarely been identified at a chemical level. This report describes a patient who developed severe, Coombs-positive hemolytic anemia on two occasions after taking the nonsteroidal anti-inflammatory drug etodolac. Studies were performed to characterize etodolac metabolites to which this patient was sensitive. METHODS Serum was tested for antibody in the presence and absence of drug using conventional methods and urine from individuals taking etodolac as a source of drug metabolites. Urinary metabolites of etodolac were identified by high-pressure liquid chromatography analysis. Glucuronide conjugates of etodolac and the 6-OH metabolite of etodolac were synthesized in a rat liver microsomal system to obtain reference standards. RESULTS The patient's serum gave only trace (+/-) reactions with normal RBCs in the presence of etodolac but reacted strongly (4+) in the presence of urine from an individual taking this drug. The active urinary metabolites were identified as etodolac glucuronide and 6-OH etodolac glucuronide. CONCLUSIONS This patient appears to have experienced acute, severe immune hemolytic anemia on two occasions because of sensitivity to the glucuronides of etodolac and 6-OH etodolac. In patients suspected of having drug-induced immune hemolytic anemia, RBC-reactive antibodies can sometimes be detected by using urine from an individual taking the implicated medication as the source of drug metabolites in in vitro reactions. For patients who present with acute immune hemolysis, a careful history of drug exposure should be taken, and, where indicated, confirmatory testing should be performed to identify the sensitizing drug and prevent inadvertent reinduction of hemolysis at a later time.