The objective was to examine the antibody responses to influenza immunization in an elderly population and the effect of prophylactic acetaminophen on adverse responses due to inactivated whole virus vaccine containing influenza A (H3N2 and H1N1) and B antigens. During the autumn of 1990, 100 patients 65 years or older were immunized and randomly allocated to receive placebo or 1,950 mg (2 x 975 mg) of acetaminophen. They recorded any local and systemic side effects over a 3-day period. Serology was performed on pre- and post (4-6 weeks) -vaccination sera. Age and gender distribution in the study were: 47% who were 75 years or older, and 61% of the patients were female. Most of the patients (97%) had pre-existing antibodies to Influenza A or B. Average peak preimmunization antibody titers were 40 to B Yamagata and A Taiwan (H1N1) and 80 for A Shanghai (H3N2). Half of each treatment group had a 4-fold or greater rise in antibody titer in response to the vaccine. Only 30% of patients immunized the previous year but 80% of those never vaccinated previously demonstrated a 4-fold or greater serological response to the vaccine. However, measurement of protection rates (HI >/= 40) before and after vaccination indicated 81.1-100% protection for the 3 viruses not influenced by treatment, gender or a history of previous vaccination. Both treatment groups had equally small numbers of patients who recorded systemic symptoms of drowsiness, myalgia, fever and chills and about 50% had arm soreness. Although about 80% of previously unimmunized adults mounted a 4-fold antibody rise to influenza vaccine antigens whereas booster effects were seen in only 30% of those immunized the previous year, protection rates were high (81-100%) after immunization and were not affected by acetaminophen treatment. Adverse effects (15% systemic and 50% local) were not ameliorated by 1950 mg of acetaminophen in these elderly patients.