The KT/V urea index (K, clearance; T, treatment time; V, volume of urea distribution) has become an established index of hemodialysis (HD) adequacy, values of KT/V less than 0.8 being associated with overt uremic toxicity. For the typical continuous ambulatory peritoneal dialysis (CAPD) regimen of 4 X 2 L exchanges/day, the equivalent KT/V approximately 0.6. Paradoxically, overt uremic toxicity is not commonly observed in CAPD patients with this typical therapy prescription. Application of the urea kinetic model demonstrates that HD and CAPD have the same time-averaged urea concentration at the same KT/V. However, as HD is an intermittent therapy, the urea concentration in HD exceeds the time-averaged concentration for about half the hours in the week. If uremic toxicity is related to the peak rather than the time-averaged urea concentration, a higher KT/V would be required in HD to achieve a peak concentration at or below the steady-state CAPD concentration. This peak concentration hypothesis predicts, based on the results of the National Cooperative Dialysis Study, that underdialysis with CAPD would occur at KT/V less than 0.4 for a protein intake of 1.1 gm/kg/day.