Patients treated with renal replacement therapy (RRT) are considered to be at higher risk for infection with hepatitis B virus (HBV). Immunoprophylaxis is therefore deemed a standard of care. Active immunization in RRT patients leads to a lower incidence of protective titers of HBV antibodies (HBAbs) than the titers seen in healthy counterparts. Our hypothesis is that, for complex reasons, active immunization is more effective in patients on peritoneal dialysis (PD) than in patients on hemodialysis (HD), and that the effectiveness of immunization depends on dialysis adequacy (Kt/V). We carried out a prospective multicenter study with an enrollment period that ran from January 1998 to December 2004. Follow-up data were analyzed as of August 2004. Inclusion criteria were an age of 18 years or older and newly indicated RRT Exclusion criteria were a history of HBV or the presence of either HBV antigen (HBAg) or HBAbs in the protective range. The choice of RRT modality (HD or PD) was based on patient preference (preceded by thorough counseling). Active immunization followed accepted guidelines for RRT patients and began after clinical and laboratory steady state had been achieved. The endpoints were the number of patients with a protective HBAb titer and the number with newly diagnosed hepatitis B. In PD patients, we calculated Kt/V on regular basis. We enrolled 211 patients, 171 of whom chose HD treatment, and 40 of whom chose PD. Positive response to immunization (defined as a serum level of HBAbs above 10 mIU/mL) was achieved in 58 HD patients (34%) and 21 PD patients (53%, p = 0.03). In subgroup of PD patients with a weekly Kt/Vgreater than 1.7 (n=28), the response rate rose to 71%--as compared with just 8% in patients with a weekly Kt/V below 1.7 (p = 0.0003). In the PD cohort as a whole, the level of HBAbs correlated with Kt/V No new cases of hepatitis B or HBAg positivity occurred in either group. From the viewpoint of successful HBV immunoprophylaxis in RRT patients, PD is the better modality choice. In PD patients, the success rate of immunoprophylaxis depends on weekly Kt/V, which we propose should be 1.7 or higher