The utility of bioelectric impedance analysis was assessed for longitudinal evaluation of body composition in two groups of uremic patients, one on CAPD and one on hemodialysis treatment, with no clinical marks of hyperhydration or infection. Nineteen CAPD patients (11 men 8 women) and 21 HD patients (12 men 9 women) were studied with bioelectric impedance analysis for a period of 12 months; total body water, fat free mass, and fat mass were calculated from bioelectric impedance analysis data of resistance and reactance at time 0 and 12 months later. No significant differences in body composition were found in the two groups at time 0 and 12 months later, with a similar trend for total body water, fat free mass, and fat mass. In CAPD, a significant increase in body weight was observed due mainly to a rise in fat mass, particularly evident in women with uremia. Bioelectric impedance analysis appears to be an instrument easily repeatable and reliable in CAPD and HD patients, reflecting at the same time body composition, the dialytic adequacy of a technique, and the patient's well being.