Atypical antipsychotics have been widely used for the management of patients with schizophrenia and other psychotic disorders. However, they may be associated with a greater risk of metabolic abnormalities than others, including weight gain, hyperlipidemia, and new-onset type 2 diabetes mellitus or diabetic ketoacidosis (DKA). We report two cases of reversible DKA and new-onset DM that developed in patients treated with atypical antipsychotics. A 42-year-old male patient with schizophrenia who was on olanzapine admitted to the hospital because of DKA. He had been taking olanzapine for 5 months. Five months before the admission, his fasting serum glucose levels were 109 m/dL. Another 34-year-old male with no previous history of diabetes mellitus was admitted to the hospital and subsequently diagnosed with DKA. The patient had been taking risperidone. Clinicians should monitor blood glucose concentrations periodically in patients taking atypical antipsychotics.