Novel antipsychotics impart substantial weight gain. Persons with bipolar disorder are frequently treated with these and other agents known to impart substantial weight gain. We sought to describe the influence of adjunctive risperidone and olanzapine on body weight, body mass index (BMI, kg/m2) and serum leptin levels over a prospective observation period of 6 months. Throughout the 6-month investigation, significant increases from baseline to end point in weight were noted with both agents; with significantly greater weight gain with olanzapine (t(10) = 2.761, P = 0.023; t(9) = 4.783, P = 0.001). Leptin levels were highly correlated with increases in weight and were significantly elevated from baseline at 4 months (r = 0.658, P < 0.05). Significant increases in weight and body mass index were apparent at 3 months (P < 0.05). The temporal association between weight increase and leptin changes does not support the notion that leptin is a primary promoter of antipsychotic-induced weight gain; however, a secondary perpetuating role cannot be ruled out.