The rising incidence of obesity in the United States has given physicians an increased role in its treatment. Although unsupervised programs can produce significant weight losses, the lack of medical supervision increases the potential for health problems. As with other lifestyle changes (for example, smoking cessation and blood pressure control), even minimal physician involvement may enhance outcome. In published clinical trials, the absence of contact with health professionals among control group participants may account in part for their poor success at weight loss or for their weight gain. Smaller trials examining the value of physician advice and encouragement among dieting patients have shown promising results. Physicians should monitor the health of obese and overweight patients during and after weight loss as is appropriate for the patient, depending on caloric levels, rate of weight loss, weight-loss goals, and intercurrent health events. Medical supervision is necessary for patients on very-low-calorie diets, for severely obese patients (body mass index > 35), and for patients with other health problems.