Weight gain is a serious side effect of antipsychotic therapy. Potential consequences of obesity include noncompliance with medications as well as significant morbidity and mortality. Because atypical antipsychotic agents are increasingly being used in adolescents, it is important to determine and further characterize the rate of weight gain in this population. This study compares weight gain associated with the use of conventional antipsychotic agents with that seen with risperidone in adolescents. This retrospective study identified 60 adolescent inpatients who were taking risperidone (n = 18), conventional neuroleptics (n = 23), or no antipsychotic medication (n = 19). Monthly height and weight measurements were recorded over a 6-month period, allowing body mass index (BMI) values to be calculated. Subjects treated with risperidone gained a mean of 8.64 kg and increased in body mass by 3.67 kg/m2 over the 6-month observation period. Those taking conventional antipsychotics gained 3.03 kg or 0.31 kg/m2 in body mass, whereas the control group actually lost weight (-1.04 kg, or -1.01 kg/m2) in the same time frame. Although both antipsychotic groups gained in weight and BMI, the risperidone-treated group gained significantly more body mass than did the conventional antipsychotic group (p = 0.0011). Gains in body weight did not correlate with dose, and concomitant medications such as stimulants and lithium did not contribute to changes in BMI.