OBJECTIVE To determine the efficacy of influenza virus vaccine in patients with systemic lupus erythematosus (SLE). METHODS The study population comprised 24 patients with SLE who received the split-virion, inactivated vaccine containing 15 micro g hemagglutinin (HA)/dose of each of A/Beijing/262/95(H1N1), A/Sydney/05/97(H3N2), and B/Harbin/07/94. Hemagglutination inhibition (HI) antibodies were tested using the HI test according to a standard World Health Organization procedure. Immune response was defined as 4-fold or greater rise in HI antibodies 6 weeks after vaccination. Geometric mean titers (GMT) were calculated to assess the immunity of the whole group. RESULTS All patients were women. Prior to vaccination, the percentage of SLE patients with protective levels of HI antibodies and the GMT of HI antibodies were similar to those of age matched healthy women. Six weeks after vaccination, 75% of the patients had immune response to at least one of the 3 antigens; 58.3% and 62.5% of the patients responded to A/Sydney/05/97(H3N2) and B/Harbin/07/94, respectively. However, only 37.5% of the patients responded to A/Beijing/262/95(H1N1). Six weeks after immunization, the SLE patients generated immune response against a mean number of 1.5 of the 3 influenza vaccines. There was a trend toward a lower immune response in patients with age > or = 50 years, prednisone dosage > or = 10 mg daily, and who used azathioprine. However, methotrexate therapy was not associated with decreased response. CONCLUSIONS The immune response to influenza vaccine of patients with SLE is lower than that seen in adults in the general population, in particular among older patients and those treated with immunosuppressive therapy.