We assessed the utility of bioelectric impedance analysis (BIA) and anthropometry for longitudinal evaluation of body composition in continuous ambulatory peritoneal dialysis (CAPD) patients. Eleven subjects were studied at the beginning of CAPD and again at regular intervals during the first 6 months of treatment. The significant weight gain that occurs in our patients is mainly due to a rise in total body water (TBW), as measured by BIA, during the first weeks of CAPD, and later on due to a body fat increase. Anthropometry seems more reliable than BIA in the evaluation of body mass, because the latter is derived from TBW in BIA. Therefore, any change in TBW that occurs in a CAPD patient necessarily causes a similar change in the fat-free mass. In our experience, only the combined use of both anthropometry and BIA allows a proper assessment of body composition in patients on CAPD.