Background. Limited data exist on the immunogenicity of the 2009 influenza A (H1N1) vaccine among immunocompromised persons, including those with human immunodeficiency virus (HIV) infection. Methods. We compared the immunogenicity and tolerability of a single dose of the monovalent 2009 influenza A (H1N1) vaccine (strain A/California/7/2009H1N1) between HIV-infected and HIV-uninfected adults 18-50 years of age. The primary end point was an antibody titer of ≥1:40 at day 28 after vaccination in those with a prevaccination level of ≤1:10, as measured by hemagglutination-inhibition assay. Geometric mean titers, influenza-like illnesses, and tolerability were also evaluated. Results. One hundred thirty-one participants were evaluated (65 HIV-infected and 66 HIV-uninfected patients), with a median age of 35 years (interquartile range, 27-42 years). HIV-infected persons had a median CD4 cell count of 581 cells/mm3 (interquartile range, 476-814 cells/mm3), and 82% were receiving antiretroviral medications. At baseline, 35 patients (27%) had antibody titers of >1:10. HIV-infected patients (29 [56%] of 52), compared with HIV-uninfected persons (35 [80%] of 44), were significantly less likely to develop an antibody response (odds ratio,.20; P 5.003). Changes in the median geometric mean titer from baseline to day 28 were also significantly lower in HIV-infected patients than in HIV-uninfected persons (75 vs 153; P 5.001). Five influenza-like illnesses occurred (2 cases in HIV-infected persons), but none was attributable to the 2009 influenza H1N1 virus. The vaccine was well tolerated in both groups. Conclusions. Despite high CD4 cell counts and receipt of antiretroviral medications, HIV-infected adults generated significantly poorer antibody responses, compared with HIV-uninfected persons. Future studies evaluating a 2-dose series or more-immunogenic influenza A (H1N1) vaccines among HIV-infected adults are needed (ClinicalTrials.gov NCT00996970).