Hepatitis B virus (HBV)-reverse seroconversion (RS) following allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a frequent late-onset complication in recipients with previous HBV infection. We followed 38 allo-HSCT recipients with previous HBV infection, and conducted posttransplant HB vaccine intervention in 13 recipients. First, we followed the recipients without any intervention (historic control) until 2003; hence, we commenced HB vaccination. Out of the patients who underwent transplantation after 2003, 13 recipients were immunized by a standard three-dose regimen after immunosuppressant cessation (vaccine group), whereas 12 recipients were observed without any intervention (nonvaccine group). Eight of the 13 historic control group recipients and 3 of the 12 nonvaccine group recipients, but none of the 13 vaccine group recipients, suffered HBV-RS. Cumulative risks of HBV-RS at 3 years post-HSCT in the historic control, nonvaccine and vaccine groups were 41%, 39%, and 0% respectively (P=.022). We therefore conclude that intervention with HB vaccines is significantly effective in preventing post-HSCT HBV-RS.