We describe the case of a 39-year-old patient with schizophrenia who developed worsening of glucose metabolism during treatment with two different atypical antipsychotics, clozapine and quetiapine. Diabetes mellitus was recognized during clozapine treatment. During quetiapine treatment, while patient was taking diabetic diet, fasting and 1-hour glucose levels and body mass index, decreased, but 2-hour glucose levels increased. This suggests that, in some patients, monitoring of only fasting glucose level and body mass index may be insufficient for detecting the glucose metabolism abnormalities. In those patients oral glucose tolerance test may be recommended. Recommendations about when and how often clinicians should administer the test do not exist in current guidelines. Further studies are needed for the elucidation of this question.