Previous studies have linked vitamin D deficiency to hypertension, dyslipidemia, diabetes mellitus, and cardiovascular disease. The aim of this study was to investigate the short-term effects of vitamin D3 supplementation on weight and glucose and lipid metabolism in antipsychotic-treated patients. A total of 19 schizophrenic or schizoaffective patients (BMI>27 kg/m2) taking atypical antipsychotics were recruited and dispensed a 2000 IU daily dose of vitamin D3. On comparing baseline with week 8 (study end) results, we found a statistically significant increase in vitamin D3 and total vitamin D levels but no statistically significant changes in weight, glucose, or lipids measurements. Patients whose vitamin D3 level at week 8 was 30 ng/ml or more achieved a significantly greater decrease in total cholesterol levels compared with those whose week 8 vitamin D3 measurement was less than 30 ng/ml. These results suggest that a randomized trial with a longer follow-up period would be helpful in further evaluating the effects of vitamin D3 on weight, lipid metabolism, and on components of metabolic syndrome in antipsychotic-treated patients.