Obesity is a serious health problem, especially in patients with long-term mental disorders. We explored the socio-demographic, psychiatric, and clinical factors that increase the risk of changing from under- or normal weight in adolescence to overweight/obese in adulthood. We found a 3.6-fold risk of weight gain in females with psychotic disorder. Other significant correlates of weight gain in males were physical inactivity, unhealthy diet, high alcohol consumption, and being single; and in females, chronic diseases, physical inactivity, high alcohol consumption, and having at least three children. These findings emphasize the importance of regular weight monitoring in clinical practice, especially in females with psychotic disorders.