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t is well-known that second generation antipsychotics (SGAs) might cause a number of metabolic side effects, including impaired glucose tolerance, high insulin resistance, weight gain and type 2 diabetes mellitus. H1 and 5-HT2c antagonisms cause increased appetite and weight gain. These side effects are reported mostly with clozapine and olanzapine among SGAs. In this paper we report a case of dramatic weight gain with quetiapine but decrease with olanzapine. 42 year-old female has been followed up in psychotic disorders outpatient clinic since 2007. She has a history of 18 years of “chronic schizophrenia” and has been hospitalized for 3 times. She had used antipsychotics such as risperidone, sulpride, amisulpride, şuphenazine, aripiprazole in various periods. She had gained 20 kg upon use of 600 mg quetiapine over a period of 3 months in 2009 and quetiapine had been stopped. In 2010, at the time of re-use of quetiapine 600 mg she had gained 48 kg within 13 months and had hyperlipidemia. Quetiapine was stopped and 10 mg olanzapine was started. She had lost 3 kg also with diet. In the interview with the patient, she strongly refused using quetiapine. Her treatment was rearranged as aripiprazole 30 mg/day, sulpride 200 mg/day, sertraline 100 mg/day, trazodone 50 mg/day. She was clinically stable with this treatment. According to the literature, clozapine and olanzapine cause the most common metabolic imbalances while risperidone and quetiapine cause moderate weight gain and increased insulin resistance. In contrast to the literature, weight gain was severe and very discomforting in this case. As a commonly used SGAs in clinical practice, weight gain and metabolic side effects of quetiapine should not be ignored.