Objective: This study was designed to compare measured and predicted thoracic gas volume (V TG ) after weight loss and to analyze the effect of body composition confounders such as waist circumference (WC) on measured V TG changes. Design: Prospective intervention study. Setting: Outpatient University Laboratory, Lisbon, Portugal. Subjects: Eighty-five overweight and obese women (body mass index ¼ 30.073.5 kg/m 2 ; age ¼ 39.075.7 years) participating in a 16-month university-based weight control program designed to increase physical activity and improve diet. Methods: Body weight (Wb), body volume (Vb), body density (Db), fat mass (FM), percent fat mass (%FM) and fat-free mass (FFM) were assessed by air-displacement plethysmography (ADP) at baseline and at post-intervention (16 months). The ADP assessment included a protocol to measure V TG and a software-based predicted V TG . Dual-energy X-ray absorptiometry (DXA) (Hologic QDR 1500) was also used to estimate FM, %FM and FFM. Maximal oxygen uptake (VO 2 max) was assessed with a modified Balke cardiopulmonary exercise testing protocol with a breath-by-breath gas analysis. Results: Significant differences between the baseline and post-weight loss intervention were observed for body weight and composition (Vb, Db, %FM, FM and FFM), and measures of V TG (measured: D ¼ 0.2 l, Po0.001; predicted: D ¼ 0.01 l, Po0.010) variables. Measured V TG change was negatively associated with the change in the WC (P ¼ 0.008), controlling for VO 2 max and age (P ¼ 0.007, P ¼ 0.511 and P ¼ 0.331). Linear regression analysis results indicated that %FM and FM using the measured and predicted V TG explained 72 and 76%, and 86 and 90% respectively, of the variance in %FM and FM changes using dual-energy x-ray absorptiometry. Conclusions: After weight loss, measured V TG increased significantly, which was partially attributed to changes is an indicator of body fat distribution such as WC. Consequently, measured and predicted V TG should not be used interchangeably when tracking changes in body composition. The mechanisms relating the reduction of an upper body fat distribution with an increase measured V TG are worthy of future investigation.