The aim was to explore weight and body mass index (BMI) changes by baseline BMI in patients completing three years of monotherapy with various first- and second-generation antipsychotics in a large cohort in a post hoc analysis of three-year observational data. Data were analyzed by antipsychotic and three baseline BMI bands: underweight/normal weight (BMI <25 kg/m²), overweight (25-30 kg/m²) and obese (>30 kg/m²). Baseline BMI was associated with subsequent weight change irrespective of the antipsychotic given. Specifically, a smaller proportion of patients gained ≥7% baseline bodyweight, and a greater proportion of patients lost ≥7% baseline bodyweight with increasing baseline BMI. For olanzapine (the antipsychotic associated with highest mean weight gain in the total drug cohort), the percentage of patients gaining ≥7% baseline weight was 45% (95% CI: 43-48) in the underweight/normal weight BMI cohort and 20% (95% CI: 15-27) in the obese BMI cohort; 7% (95% CI: 6-8) of the underweight/normal cohort and 19% (95% CI: 13-27) of the obese cohort lost ≥7% baseline weight. BMI has an association with the likelihood of weight gain or loss and should be considered in analyses of antipsychotic weight change.