A new device based on the plethysmographic measurement of body volume has been developed for the purpose of estimating human body composition. The device, the BOD POD Body Composition System, uses the relationship between pressure and volume to derive the body volume of a subject seated inside a fiberglass chamber. Derivation of body volume, together with measurement of body mass, permits calculation of body density and subsequent estimation of percent fat and fat-free mass. Critical issues which have hampered prior plethysmographic approaches are discussed. The present system's ability to measure the volume of inanimate objects was evaluated for accuracy, reliability, and linearity. Twenty successive tests of a known volume (50,039 ml) on two separate days produced values of 50,037 +/- 12.7 ml and 50,030 +/- 13.5 ml (mean +/- SD) for each day, respectively. The CV for these series were 0.025% and 0.027%. Further testing across a wide range of volumes approximating human size (25-150 1) produced the following regression equation where y = measured volume (1) and x = actual volume (1): y = 0.9998x - 0.0274, r2 = 1.0, SEE = 0.004 1. The resultant device is likely to enhance opportunities for the quick, simple and noninvasive measurement of body composition for both research and clinical applications.