Obesity is a condition affecting one-third of the adult U.S. population and is a major risk factor for the development of Type 2 diabetes, hyperlipidemia, hypertension, and other cardiovascular and respiratory disorders. Individuals with the onset of obesity during childhood or adolescence are at increased risk for obesity-related, comorbid conditions, both during adolescence and later in life. African American girls and women are at particular risk for obesity, and have substantial rates of obesity-related morbidity and mortality. Further, African American adult women have a less satisfactory response to many therapeutic approaches used to treat obesity. At present, there are no medical therapies proven effective for the amelioration of severe obesity in children or adolescents. One medication that may have a favorable risk-benefit ratio in pediatric populations is orlistat (Xenical(Trademark), Hoffmann LaRoche). Orlistat acts by inhibiting gastrointestinal lipases, interfering with the absorption of approximately 1/3 of ingested dietary fat. Orlistat appears to be effective for reducing weight and obesity-associated comorbidities in obese adults. We propose to determine the safety, tolerability, and efficacy of orlistat in 12-17 year-old severely obese African American and Caucasian children and adolescents who have one or more obesity-related comorbidity (hypertension, hyperlipidemia, sleep apnea, hepatic steatosis, insulin-resistance, impaired glucose tolerance, or Type 2 diabetes). Under this protocol, we have conducted an open-label pilot study of orlistat in twenty subjects, suggesting orlistat has a similar side effect profile in adolescents as in adults. We wish to determine the safety and efficacy of orlistat in reducing obesity-related comorbidities using a randomized, double-blind, placebo-controlled clinical trial. All study participants will be enrolled in a psycho-educational weight loss program that includes nutrition education, cognitive-behavioral self-monitoring strategies, and promotion of physical activity. We will also study the effects of orlistat on fat preferences, and study the influence of genetic variables on energy expenditure and weight loss during treatment. A group of healthy, non-overweight children and adolescents will complete questionnaires and exercise studies as a control group for interpretation of results in overweight children and adolescents, but will not undergo phlebotomy or receive any medication.