In Maryland, 2,046 people were treated with clozapine through Medicaid or the Maryland Pharmacy Assistance Program between 1990 and 2000 and were registered with the Clozapine Authorization and Monitoring Program. Three cases of lethal diabetic ketoacidosis occurred during clozapine treatment (0.15%), as noted by death certificates. Two of the three patients were male, and the mean age at death was 38.0 years (SD=6.1). All three people had schizophrenia and were being treated in outpatient settings. Concomitant psychiatric medications included sertraline, divalproex, fluoxetine, and methylphenidate. None of the patients had a diagnosis of diabetes mellitus or was being treated for this disorder. Weight, smoking status, and family history were unknown. All had been taking clozapine for over 1 year before death (25.5 months, 14.5 months, and 59.5 months; mean=33.2 months). To our knowledge, this is the first report of lethal diabetic ketoacidosis during long-term clozapine treatment.