The assessment of human body composition is important for both clinical investigation and medical practice, and many methods are available. Most of these methods are based upon a model in which the body consists of two chemically distinct compartments, fat mass and fatfree mass. The pioneering work of Behnke et al.3) and Keys and Brozek10) resulted in the establishment of densitometry as the criterion by which all other indirect methods were evaluated as a basis for body composition studies. Over the past decade a number of new methods for assessment of human body compositionhave been developed, e. g., neutronactivation analysis2), computed tom ography41 and NMR imaging17), total body electrical conductivity (TOBEC) 27), and bioelectrical impedance 12, 20), However only a few of these are applicable to humansubjects because most of the methods are inconvenient and require active cooperation of the subjects. Because of its convenience and low cost, bioelectrical impedance measurement (BIM) has become a popular method for determining body composition in human subjects. Previous studies have demonstrated a strong relationship between BIM and total body water(by isotope dilution) 8,15', fatfree mass (FFM, by densitometry) 19, 20, 23, 25),percentage body fat (%FAT) 19, 20) in human subjects. However, most BIM studies have been performed on weight-stable subjects. In addition, none of the previous studies evaluated the sensitivity of the BIM method for assessing changes in body composition resulting from a physical exercise and diet regimen. Ross et al.22) and Kushner et al.16) suggested that BIM was a useful clinical method for measuring changes in body composition. On the other hand, Deurenberg et al.s’ and Vazquez and Janosky29’ suggested that the BIM method did not appear to be a valid approach for measuring the small changes in FFM that occur duringtreatment of obesity. Thus, the validity of BIM for measuring changes in body composition during weight reduction is controversial. © 1992, The Japanese Society of Physical Fitness and Sports Medicine. All rights reserved.