Olanzapine, an atypical antipsychotic, has been widely used for the treatment of schizophrenia and bipolar disease. Although olanzapine is less associated with extrapyramidal symptoms and neuroleptic malignant syndrome compared to existing typical antipsychotics, the use of this drug has a problematic side effect of weight gain, which may cause metabolic syndrome such as type 2 diabetes. However, there are few hospitals practicing body weight monitoring of the patients on olanzapine or other atypical antipsychotics. The goal of this study was to identify the incidence and severity of weight gain associated with the use of the drug in Korea. We performed body weight monitoring of the patients who were on the drug in a hospital setting. Mean of the weight gain (as of one-month-transformation) was 4.33 and 3.39 kg for the male and female patients, respectively. The incidence in the young patients was higher than that observed in the old patients, and the severity was the highest in patients in their thirties followed by twenties or younger. This result suggests that the pattern of the weight gain associated with the use of olanzapine in Korea is similar to the reports performed and documented in US and European countries. Therefore, it appears that healthcare professionals in Korea should also watch on the weight gain issue in patients who are on olanzapine or other atypical antipsychotics.