The capacity of patients infected with human immunodeficiency virus (HIV) to develop an adequate antibody response to influenza vaccination in relation to the CD4 cell count has been studied in a prospective study. A total of 73 subjects (54 HIV-infected patients and 19 healthy control persons) were vaccinated with influenza subunit vaccine containing 15 mug hemagglutinin of each of the following strains: A/Beijing/353/89(H3N2), A/Singapore/6/86(H1N1), B/Panama/45/90, and B/Beijing/1/87. Hemagglutinin inhibition (HI) antibody titers were determined prior to vaccination, 3 weeks afterwards, and at the end of the influenza season. The percentage of subjects with HI antibody titers above the assumed protective level was significantly lower in the HIV-infected patients for all 4 vaccine strains compared with those in the control group (7-26% and 42-74%, respectively). There was an association between CD4 cell count and antibody response to the B/Panama strain only. The serologic response to tetravalent subunit influenza vaccine is severely impaired in the majority of HIV-infected patients compared with control subjects. The results of this study challenges the recommendation to vaccinate HIV-infected patients.