The aim of this 6-month observational study was to examine which clinical, eating- and lifestyle-related factors were associated with weight gain in patients initiating or switching to oral olanzapine for the treatment of schizophrenia or bipolar mania. A total of 622 outpatients in four countries (China, Mexico, Romania, Taiwan) were assessed at monthly intervals for up to 6 months. Mixed model repeated-measures analysis, adjusted for baseline weight, was used to identify which factors were associated with weight gain during olanzapine therapy. After 6 months of therapy, the LS mean weight change was +4.1 kg and 43.9% of the patients had significant (> or = 7%) weight gain. Early significant weight gain after 2 months of therapy occurred in 23.4% of the patients and these patients gained significantly more weight overall. Ten factors were associated with weight gain during 6 months of olanzapine therapy in an exploratory multivariate analysis: country, housing conditions, stronger appetite, excessive amount of food needed to feel full, eating until uncomfortably full, thoughts preoccupied with food, meal location, increased meal frequency, evening snack consumption, and a lower amount of vigorous exercise. These results indicate that the influence of environmental, eating- and lifestyle-related factors should be considered when assessing weight gain during olanzapine therapy.