We assessed the utility of bioelectric impedance plethysmography (BIA) for measuring acute volume changes and body composition in patients with renal disease. Chronic hemodialysis and peritoneal dialysis patients were evaluated before and after dialysis. Total body water, as measured by BIA, predicts body weight. Acute changes in body water, induced by dialysis, are not predicted well by data derived from BIA. Body fat (%) and fat-free mass (%), determined by anthropometry, are predicted well by body composition data derived by BIA. BIA is useful for assessing body composition in this population, but must be cautiously applied to the measurement of acute volume changes.