Little data are available regarding the safety and immunologic response to pandemic H1N1 influenza vaccine in recipients of allogeneic hematopoietic stem cell transplantation (HSCT). We measured serum antibody titers against A/California/7/2009 H1N1 using a hemagglutination inhibition assay in 82 allogeneic HSCT recipients who received the 2009 H1N1 vaccine between November 2009 and January 2010 after it became available at our institution. The median time between HSCT and vaccination was 19 months (range, 2.5-94 months), and the median time from vaccination to specimen collection was 56 days (range, 14-140 days). Seroprotective antibody titers (hemagglutination inhibition titer ≥1:40) against 2009 H1N1 influenza A virus were detected in 51% of patients. The presence of chronic graft-versus-host disease and type of conditioning regimen did not affect the rate of detection of seroprotective titers after vaccination. Patients were more likely to have a seroprotective titer the farther away from HSCT they were (adjusted odds ratio, 1.79 per year; 95% confidence interval, 1.12-2.85). Rituximab administration in the year before vaccination was associated with a lack of seroprotective titer (adjusted odds ratio, 0.11; 95% confidence interval, 0.01-0.97). The vaccine was safe and well tolerated. Strategies are needed to improve the influenza vaccine response in this population, especially those receiving immunotherapy.