The aim of this study was to assess the prevalence of weight gain during long-term treatment with olanzapine under routine conditions. Weight changes in a sample of 27 outpatients with schizophrenic or schizoaffective disorder were assessed over a mean duration of treatment of 22 months (6–42 months). Treatment with olanzapine was started anew or switched from a conventional antipsychotic drug. Eighteen (66.7%) patients gained more than 7% of their initial body weight. Mean weight gain was 9.2 kg over the study period for the first year 7.7 kg, and only 1.7 kg in the second year. Due to some patients losing weight in the second year, the latter finding obscures the fact that more than 50% of patients gained considerable weight also during the second year (4.8 kg). Weight gain per month was significantly higher in patients with lower body mass index, yet the highest weight gain was found in the most obese patient. Weight gain under olanzapine is a serious concern, and should be monitored closely and countered by active measures.