Objective: Acute exacerbations adversely affect the quality of life and prognosis of patients with chronic obstructive lung disease (COPD). Prevention of future exacerbations is extremely important, especially for elderly patients. In this study, we evaluated the efficacy of influenza vaccine for acute exacerbation of COPD in elderly patients. Methods: A prospective cohort study was conducted among 289 patients over 65 years of age with COPD (FEV₁/FCV<0.70) during the 2001-2002 influenza season. Background data, outpatient visits for wheezing and hospitalizations were compared between the vaccinated group (n=189) and the unvaccinated group (n=100). Results: The number of patients who visited hospital for wheezing was 11 of 189 (5.8%) in the vaccinated group and 23 of 100 (23%) in the unvaccinated group (RRR: relative risk reduction 74.7%, 95%CI: confidence interval 0.51-0.87). The number of hospitalizations for pneumonia was 8 of 189 (4.2%) in the vaccinated group and 14 of 100 (14%) in the unvaccinated group (RRR 69.8%, 95%CI: 0.32-0.87). The costs of hospitalization were lower in the vaccinated group with direct savings of 91,525 yen per patient. Conclusions: For elderly COPD patients, influenza vaccine decreases acute exacerbation due to pneumonia and bronchoconstriction, and also may minimize the costs of hospitalization.