A population-based cohort study was conducted during the 2003-2004 season to examine the effectiveness of influenza vaccine among community-dwelling elderly. The subjects consisted of 4787 elderly, ranging from 65 to 79 years. We either interviewed the elderly directly or their families regarding acute febrile illness, hospital visits, hospitalization and death by telephone every month. The vaccination status and physician-diagnosed clinical influenza (hereinafter referred as clinical influenza) were determined based on data obtained from the city office and hospitals, respectively. Influenza-like illness (ILI) was defined as an acute febrile illness (> or = 38.5 degrees C) during the epidemic period. After adjusting for confounders, vaccination decreased ILI significantly (odds ratio [OR], 0.38; 95% confidence interval [CI], 0.17-0.85), but not clinical influenza (OR, 0.76; 95% CI, 0.28-2.06). The results were inconclusive for preventing hospitalization for influenza or pneumonia (OR, 0.37; 95% CI, 0.09-1.47) and death (OR, 3.68; 95% CI, 0.75-18.12), due to the inadequate sample size. In conclusion, the influenza vaccination was thus found to be associated with a decreased ILI during the epidemic period in community-dwelling elderly.