With the broad use of atypical anti-psychotics, altered glucose metabolism has become an item of concern to clinicians and patients. Among the atypical anti-psychotics, clozapine and olanzapine are associated with a relatively high incidence of diabetic ketoacidosis and newly developed diabetes. We report a case of diabetic ketoacidosis in a patient with long-term clozapine therapy. A 35-year-old male with schizophrenia, treated with clozapine for 9 years was admitted into hospital because of comatous mentality. Although never diagnosed with diabetes before, his clinical features were consistent with diabetic ketoacidosis and shock. The patient's serum amylase and lipase levels were elevated and an abdominal computed tomography showed peripancreatic fat infiltration, suggesting the possibility of acute pancreatitis. The patient's serum glucose levels normalized shortly after clozapine treatment. Moreover, the patient ceased all glucose lowering agents upon hospital discharge, and maintained normal blood glucose levels thereafter. As observed in this case, clinicians should carefully screen and monitor blood glucose levels and other clinical parameters in patients treated with atypical anti-psychotics.