Objective: Patients with schizophrenia and schizooffective disorder suffer from increased rates of weight gain that might lead to severe medical problems. This study aimed to investigate the prevalence of overweight and obesity in patients with schizophrenia and schizoaffective disorders and to determine the change of it in three years. Method: Body Mass Index (BMI) were assessed in patients with schizophrenia and schizooffective disorder at the beginning and with a follow-up interval of six months in a psyhosis outpatient clinic between the years 2004 and 2008. The relationship between BMI and sex, age, illness duration, family history of obesity and diabetes were investigated. Results: A total of 492 patients (n= 443 for schizophrenia; n=49 for schizoaffective disorder) were assessed in a four-year period. The mean age of the patients was 35.0 (SD=11.3) years, and the mean BMI was 27,1 (SD=5.21 kg /m(2). The prevalence of overweight and obesity were 45.5% and 24.4%, respectively. Family history of obesity and diabetes were 34.9% and 23.9%, respectively. BMI were continuously increasing in each evaluation and were significantly higher in month 24th, 30th, and 36th. Positive correlation at the first, second, and third evaluation between BMI and age and duration of illness disappeared in the evaluation of 24th month and further. A significant difference between men and women also disappeared after the evaluation of 24th month. BMI was higher in patients who have positive family history of obesity or diabetes than those who have not (p=0.001). Conclusion: The problem of weight gain (BMI >= 25) is prevalent (69.9%) in patients with schizophrenia and schizoaffective disorders, Family history of obesity and diabetes were found to be risk factors for gaining weight. This situation should not be overlooked and further multidisciplinary studies are needed for this severe problem in psychosis treatment.