Kinetic analysis was performed in all 58 patients undergoing standard CAPD. The urea distribution volume was estimated from anthropomorphic measurements (Watson formulae). Normalized protein catabolic rate (NPCR), daily protein leak (PL), urea and creatinine Kt/Vs, clearances and peritoneal mass transfer coefficients (Kp) were calculated from measurements on serum, 24-h urine and PD fluid effluent. The mean total (renal+PD) daily creatinine and urea Kt/Vs (KT/V) were 0.31 (range 0.15-0.79) and 0.31 (0.18-0.65). There was no relationship between KT/V and serum urea or Kp. The strongest determinant of the urea KT/V was the residual renal urea clearance (KrU)(R = 0.79, P < 0.001) which decreased with time on dialysis (R = -0.38, P < 0.005). There was a significant correlation between the hospital admissions per year and both the urea and creatinine KT/V and KrU (R = -0.30, -0.32, P < 0.05). Patients with urea KT/V < 0.25 (n = 22) had more hospital admissions/year than those with KT/V > 0.25 (mean of 2.6 versus 1.5, P < 0.05). NPCR correlated with urea KT/V (R = 0.62, P < 0.001) but not with serum albumin or the PL. Patients identified by UKM to be less well dialysed have a lower residual renal function and are more likely to be hospitalized. Undernutrition in CAPD patients appears to be related to underdialysis rather than protein loss.