Objective: The purpose of this study was to validate the use of the leg-to-leg bioelectrical impedance analysis (BIA) system in assessing change in body composition over 32 weeks in overweight and obese women participating in a community weight management program. Design: Intervention, with subjects prescribed an energy-restriction diet and exercise program for 32 weeks and body composition measured pre-study and after 12 and 32 weeks. Subjects and Setting: Overweight and obese premenopausal women (n=201) with no overt disease were recruited at six sites into community-based weight loss programs. One hundred and twenty-four women completed all aspects of the study. Intervention: Energy intake was set at 0.8 × resting metabolic rate (RMR) for weeks 1 through 12, 1.0 × RMR for weeks 13 through 20 and 1.2 × RMR for weeks 21 through 32. Energy intake was based on a food exchange table, with the number of food exchanges adjusted to encourage a percent distribution of 55% carbohydrate, 30% fat and 15% protein. Subjects increased their daily walking distance by 3.2 km above pre-study levels. Measures of Outcome: Underwater weighing, seven skinfolds, and leg-to-leg BIA tests were used to assess body composition. Results: A 3 × 3 repeated measures ANOVA revealed no significant difference in detecting change in FFM at 12 and 32 weeks among underwater weighing, BIA and skinfold, (F(4,492)= 1.73, p= 0.141) (decrease in FFM of 1.0±3.3 kg, 1.7±2.2 kg, and 1.4±3.3 kg respectively, 32 weeks). Conclusions: The leg-to-leg BIA system provides a valid measure of body composition change in overweight premenopausal women during a 32-week community-based weight loss program.