In November and December 1993, 49 children, ages 4 to 20, suffering from acute lymphoblastic leukemia, were vaccinated against influenza in the Department of Paediatric Haematology and Oncology, Medical Academy in Warsaw. These patients were vaccinated either in the course of maintenance treatment or after treatment. Each dose of Wyeth USA subunit trivalent influenza vaccine contained 15 micrograms of hemagglutinin of strains recommended for that season. The level of antibody production was determined in pre- and post vaccination sera in the group of children with leukemia and the control group. It was determined that in the investigated group, the GMT increased more than four times for hemagglutinins H1N1 and H3N2. A somewhat lower increase was observed in case of hemagglutinin HB. The proportion of subjects protected after vaccination was 35% for hemagglutinin H1N1, 76% for H3N2 and 100% for HB. The response rate was 33% for hemagglutinin H1N1, 47% for H3N2 and 45% for HB. In the control group the proportion of subjects protected and the response rate were very low. The results show the significant immunological efficacy of the vaccine used in the vaccination against influenza in high risk groups.