BACKGROUND: Increased prevalence of adolescent obesity requires effective treatment options beyond behavior therapy. OBJECTIVE: To see whether sibutramine reduced weight more than placebo in obese adolescents who were receiving a behavior therapy program. DESIGN: 12-month, 3:1 randomized, double-blind trial conducted from July 2000 to February 2002. SETTING: 33 U.S. outpatient clinics. PARTICIPANTS: 498 participants 12 to 16 years of age with a body mass index (BMI) that was at least 2 units more than the U.S. weighted mean of the 95th percentile based on age and sex, to the upper limit of 44 kg/m2. Interventions: Site-specific behavior therapy plus 10 mg of sibutramine or placebo. Blinded study medication dose was uptitrated to 15 mg or placebo at month 6 if initial BMI was not reduced by 10%. MEASUREMENTS: Body mass index, waist circumference, body weight, fasting lipid and glycemic variables, safety, and tolerability. RESULTS: Seventy-six percent of patients in the sibutramine group and 62% of patients in the placebo group completed the study. The estimated mean treatment group difference at month 12 (linear mixed-effects model) favored sibutramine for change from baseline in BMI (-2.9 kg/m2 [95% CI, -3.5 to -2.2 kg/m2]) and body weight (-8.4 kg [CI, -9.7 to -7.2 kg]) (P < 0.001 for both). The sibutramine group had greater improvements in triglyceride levels, high-density lipoprotein cholesterol levels, insulin levels, and insulin sensitivity (P < or = 0.001 for all). The rate of tachycardia was greater with sibutramine vs. placebo (12.5% vs. 6.2%; difference, 6.3 percentage points [CI, 1.0 to 11.7 percentage points]) but did not lead to increased withdrawal (2.4% vs. 1.5%; difference, 0.9 percentage point [CI, -1.7 to 3.5 percentage points]). LIMITATIONS: The 1-year study duration precluded assessment of long-term weight maintenance and putative health benefits and harms, and 24% and 38% of the sibutramine and placebo groups, respectively, did not complete follow-up. CONCLUSIONS: Sibutramine added to a behavior therapy program reduced BMI and body weight more than placebo and improved the profile of several metabolic risk factors in obese adolescents.