Fifty-two children, aged less than 5 years, with chronic lung disease or congenital heart disease were entered into a two-centre open study to determine the immunogenicity and tolerability of Influvac, a trivalent influenza sub-unit vaccine. Seroresponses were determined following two intramuscular vaccinations with 0.25 ml of Influvac, four weeks apart. Any local or systemic reaction was sought. Seroresponses were age and antigen specific, with children older than 9 months showing better seroresponses to all three antigens. Both A/Taiwan and B/Panama strains met all efficacy criteria. A/Shangdong met two of the three criteria: seroconversion and mean geometric titre increase. Local (23%) and systemic (48%) reactions following either of the two vaccinations were minor in nature and resolved within a few days. The vaccine induced a strong antibody response against all three haemagglutinin antigens and was well tolerated. The incidence of local and systemic reactions was comparable with those reported in healthy adults.