OBJECTIVE--To determine the efficacy of influenza vaccination in elderly people. DESIGN--Randomized double-blind placebo-controlled trial. SETTING--Fifteen family practices in the Netherlands during influenza season 1991-1992. PARTICIPANTS--A total of 1838 subjects aged 60 years or older, not known as belonging to those high-risk groups in which vaccination was previously given. INTERVENTION--Purified split-virion vaccine containing A/Singapore/6/86(H1N1), A/Beijing/353/89(H3N2), B/Beijing/1/87, and B/Panama/45/90 (n = 927) or intramuscular placebo containing physiological saline solution (n = 911). MAIN OUTCOME MEASURES--Patients presenting with influenzalike illness up to 5 months after vaccination; self-reported influenza in postal questionnaires 10 weeks and 5 months after vaccination; serological influenza (fourfold increase of antibody titer between 3 weeks and 5 months after vaccination). RESULTS--The incidence of serological influenza was 4% in the vaccine group and 9% in the placebo group (relative risk [RR], 0.50; 95% confidence interval [CI], 0.35 to 0.61). The incidences of clinical influenza were 2% and 3%, respectively (RR, 0.53; 95% CI, 0.39 to 0.73). The effect was strongest for the combination of serological and clinical influenza (RR, 0.42; 95% CI, 0.23 to 0.74). The effect was less pronounced for self-reported influenza. CONCLUSION--In the elderly, influenza vaccination may halve the incidence of serological and clinical influenza (in periods of antigenic drift).