OBJECTIVE Patients with moderate and severe obesity, because of their physical size, often cannot be evaluated with conventional body composition measurement systems. The BOD POD air displacement plethysmography (ADP) system can accommodate a large body volume and may provide an opportunity for measuring body density (D(b)) in obese subjects. D(b) can be used in two- or three-compartment body composition models for estimating total body fat in patients with severe obesity. The purpose of this study was to compare D(b) measured by ADP to D(b) measured by underwater weighing (UWW) in subjects ranging from normal weight to severely obese. METHODS D(b) was measured with UWW and BOD POD in 123 subjects (89 men and 34 women; age, 46.5 +/- 16.9 years; BMI, 31.5 +/- 7.3 kg/m2); 15, 70, and 10 subjects were overweight (25 < or = BMI < 30 kg/m2), obese (30 < or = BMI < 40 kg/m2), and severely obese (BMI > or = 40 kg/m2), respectively. RESULTS There was a strong correlation between D(b) (kilograms per liter) measured by UWW and ADP (r = 0.94, standard error of the estimate = 0.0073 kg/L, p < 0.001). Similarly, percent fat estimates from UWW and ADP using the two-compartment Siri equation were highly correlated (r = 0.94, standard error of the estimate = 3.58%, p < 0.001). Bland-Altman analysis showed no significant bias between D(b) measured by UWW and ADP. After controlling for D(b) measured by ADP, no additional between-subject variation in D(b) by UWW was accounted for by subject age, sex, or BMI. CONCLUSIONS Body density, an important physical property used in human body composition models, can be accurately measured by ADP in overweight and obese subjects.