Influenza viruses can cause worldwide epidemics and are important causes of mortality and morbidity due to pulmonary diseases. The best method of prevention is to vaccinate people each year before the influenza season. The purpose of this study was to evaluate ELISA in determining seroconversion following vaccination against influenza virus. Prior to 1994-95 influenza season, 50 patients suffering from chronic obstructive pulmonary diseases and asthma were vaccinated with trivalent influenza split virion vaccine. To detect seroconversion, antihemagglutinin IgA, IgM and IgG responses and their differences with time were followed with ELISA and sensitivity and specificity of ELISA were compared with hemagglutination inhibition assay. Seroconversions of IgG were found to be 52 % for H3N2 , 64 % for H1N1, and 40 % for B virus in the 6th month. Sensitivity was 76 %, specificity was 88 % in inhouse ELISA. A significant increase in antihemagglutinin IgG antibodies after vaccination was determined at the end of 6 months (p<0.005). As a result, vaccination with influenza virus vaccine caused an increase in the percentage of patients having preventive HI antibody levels. As ELISA is not time-consuming and has considerable sensitivity and specificity, it may be used for detection of seroconversion in vaccinees as well as for follow-up of influenza infection in the community.