A threat to survival in renal failure, malnutrition in continuous ambulatory peritoneal dialysis patients (CAPD) is often occult as CAPD patients often gain weight masking actual protein malnutrition. Bioelectrical impedance (BEI) accurately assesses body composition in CAPD patients and uncovers subtle changes in lean body mass (LBM) that escape indirect anthropometric detection. Segregating parameters of body composition is crucial to nutritional management of CAPD patients in whom fat may account for overall weight gain. While both skin-fold methods and BEI correctly distinguished thin and overweight patients in terms of fat mass, only BEI accurately segregated these patients by LBM (P = 0.007). Serial weights of 39 CAPD patients followed longitudinally for three or more months did not correlate with BEI-measured changes in LBM. LBM was lost in 49% of patients as determined by BEI, while serial weights detected a loss of LBM in 36% of these patients. Strikingly, by serial weights, 64% of patients demonstrated weight gain; however, in 24% of these an actual loss of LBM was demonstrated by BEI. BEI provides specific quantitation of LBM in CAPD patients with changing body habitus and unrecognized nutritional derangement.