OBJECTIVE: To investigate the effect of diet alone (D), exercise alone (E), and a combination of diet and exercise (DE) on body weight, body composition, energy intake, blood pressure, serum lipid and leptin levels, and fitness levels in mildly obese sedentary women and men. DESIGN: The three interventions were compared in a randomized longitudinal study design. The exercise programs were supervised for six months, after which participants in E and DE were provided with exercise equipment to take home. SUBJECTS: 90 adult overweight women and men (age: 44.2 +/- 7.2 years; BMI = 30.5 +/- 2.7 kg/m(2)). MEASUREMENTS: Body weight, body composition, waist and hip circumferences, blood pressure, serum lipid levels, and fitness levels were evaluated at 0, 3, 6, 9, and 12 months. Serum leptin concentrations were measured at 0 and 6 months only. RESULTS: At 6 and 9 months in women, and 9 months in men, DE demonstrated a significant loss of body weight compared to both D and E (p < 0.05). Serum leptin levels significantly decreased from baseline to 6 months in women in D (p = 0.05) and DE (p = 0.0003) and men in E (p = 0.038). At one year, no significant differences existed among groups in any of the measures. CONCLUSION: A combination of diet and exercise resulted in a significant decrease in body weight in women and men; but this decrease was not maintained at one year follow-up. Serum leptin concentrations showed significant within-group decreases, but were not different among groups. A supervised diet and exercise program is effective for weight loss; however, once intensive participant-investigator and participant-participant contact is discontinued, weight regain ensures.