Obese subjects who undergo gastrointestinal surgery for weight reduction have relatively greater loss of body cell mass (BCM) than of extracellular fluid. Although the mechanism is uncertain, an implication is that weight-reduced, surgically treated obese subjects are relatively overhydrated. The present investigation explored whether diet-treated postobese (PO) subjects also are relatively overhydrated. Ten PO participants were recruited over a two-year period and matched to never-obese (NO) people by age (+/- 4 years), weight (+/- 3 kg), height (+/- 5 cm), race, and gender. PO participants had lost > or = 18.5 kg and had maintained this loss for 2-16 years. Body density (Db), total body water (TBW), and bone mineral content (BMC) were measured by hydrodensitometry, tritium dilution, and dual-energy X-ray absorptiometry, respectively. Fat and fat-free mass (FFM) were derived using a four-compartment model. BCM was calculated from total body potassium (TBK) measured with a whole body counter. The results indicate that PO subjects were relatively overhydrated and had reduced BCM compared to NO subjects (p < 0.05). These observations are important for interpreting body weight, body composition, and metabolic data of obese subjects who lost weight and maintained their weight loss over the long term.