The effectiveness of influenza vaccination in preventing serious illness and death was determined in an elderly population during the influenza epidemic of was determined in an elderly population during the influenza epidemic of was determined in an elderly population during the influenza epidemic of 1989-90. A retrospective cohort study was carried out using computerized general practitioner records on nearly 10,000 patients aged 55 years and over. After adjustment for potential confounding factors, recent immunization was found to have a protective effect of 75% (95% confidence intervals: 21-92%) against death. Protection did not appear to vary with either age or the presence of underlying chronic disease. As the complications of influenza are most common in those with underlying chronic disease, the study findings are consistent with the recommended policy for the use of influenza vaccine in the UK. Further work is necessary to determine the cost-effectiveness of extending immunization to other groups.