Hepatitis A remains a serious vaccine-preventable disease for HIV patients. We tested 897 HIV-infected men having sex with men (MSM) for antibodies against hepatitis A virus (anti-HAV) and measured the geometric mean antibody titres (GMTs) in a group of patients who received a hepatitis A vaccine and in patients with immunity to HAV due to infection in childhood. In all, 320 patients (35%) had positive anti-HAV antibodies. Multivariate analysis showed that only age (p 0.001) and ethnic origin (OR 20.029, p 0.001) had a statistically significant effect on the presence of antibodies. In addition, age was a fairly sensitive (68.4%) and specific (64.2%) marker, patients being separated by the 36.5 years cut-off point. The response rate of patients who get vaccinated (n 383), one month following the administration of the second dose of the vaccine, was 76%. The GMT of the vaccinated patients was 305 mIU/ml versus 7105 mIU/ml of patients with past infection. The vast majority of HIV-infected MSM patients in Greece is susceptible to HAV. Immunity to HAV in newly vaccinated patients, unlike patients with natural immunity, is low and probably requires monitoring.