OBJECTIVE--This study aimed to determine if patients with HIV infection can develop a significant antibody response to influenza immunisation, and whether such immunisation is detrimental to the progression of their HIV illness. DESIGN--Prospective, non-randomised study. METHODS--The titres of antibody response to influenza immunisation were determined in 44 asymptomatic HIV positive men and compared with 16 HIV presumed seronegative controls. The magnitude of response were correlated with patients' CD4 lymphocyte counts. The rate of CD4 lymphocyte count decline over a median of 12 months before and after immunisation were also evaluated. RESULTS--Thirty-two of the 44 HIV positive subjects (73%) were able to mount a four-fold or greater response to at least one of the influenza strains, and 14 of them (32%) did so to all four strains. In comparison, 15 of the 16 controls (93.5%) had a four-fold or greater response to at least one, and six of them (38%) to all influenza strains, which was not significantly different from the HIV positive group. The magnitude of increase in antibody titre was not significantly different between the two groups either. CD4 lymphocyte count change for a median of 12 months after immunisation was not different from a median of 12 months before immunisation. CONCLUSIONS--Asymptomatic HIV infected patients are able to mount antibody response to influenza immunisation, which appears to be safe in respect to HIV illness progression in the short term.