Olanzapine treatment has been associated with clinically meaningful weight increases, hypertriglyceridemia, insulin resistance, and diabetes mellitus. There are few options for olanzapine responders who fail other antipsychotic agents. Aripiprazole is a potent (high-affinity) partial agonist at D2 and 5-HT1A receptors and a potent antagonist at 5-HT2A receptor and is associated with less weight gain than olanzapine. We report the results of a 10-week placebo-controlled, double-blind crossover study that examined 15 mg/d aripiprazole's effects on weight, lipids, glucose metabolism, and psychopathology in overweight and obese schizophrenia and schizoaffective disorder subjects treated with a stable dose of olanzapine. During the 4 weeks of aripiprazole treatment, there were significant decreases in weight (P = 0.003) and body mass index (P = 0.004) compared with placebo. Total serum cholesterol (P = 0.208), high-density lipoprotein cholesterol (HDL-C; P = 0.99), HDL-2 (P = 0.08), HDL-3 (P = 0.495), and low-density lipoprotein cholesterol (P = 0.665) did not change significantly comparing aripiprazole treatment to placebo treatment. However, total serum triglycerides (P = 0.001), total very low-density lipoprotein cholesterol (VLDL-C; P = 0.01), and VLDL-1C and VLDL-2C (P = 0.012) decreased significantly during the aripiprazole treatment phase. The VLDL-3C tended lower during aripiprazole, but the decrease was not significant (P = 0.062). There was a decrease in C-reactive protein comparing aripiprazole treatment to placebo, although it did not reach significance (P = 0.087). The addition of aripiprazole to a stable dose of olanzapine was well tolerated and resulted in significant improvements on several outcome measures that predict risk for medical morbidity.