Influenza vaccine is recommended for HIV-1-infected patients. The present prospective study was conducted to evaluate the clinical efficacy and immunologic responses to the vaccine. From November 1 to December 27, 2002, 262 HIV-1-infected patients received a trivalent influenza subunit vaccine, whereas 66 did not. Influenza illness occurred in 16 vaccinated and 14 nonvaccinated patients (incidence= 6.1%[95% confidence interval (CI): 4%-10%] in vaccinated vs. 21.2%[CI: 13%-35%] in nonvaccinated persons, P< 0.001; relative risk= 0.29 [CI: 0.14-0.55]). Influenza vaccine provided clinically effective protection against influenza illness in HIV-1-infected patients. In baseline antibody-negative patients, anti-H1 and anti-H3 antibody responses to the vaccination were significant in those patients with a CD4 count> 200 cells/μL compared with those with a CD4 count< 200 cells/μL (P< 0.05). In contrast, in baseline antibody-positive patients, good antibody responses were observed irrespective of CD4 counts, like the healthy controls. Based on these results, annual vaccination is recommended. Specific CD4 responses correlated with HIV-1 viral load (VL), especially in patients treated with highly active antiretroviral therapy (HAART) compared with those without HAART (P< 0.01), although the clinical efficacy did not correlate with HIV-1 VL. HAART may enhance the immunologic efficacy of influenza vaccine.