Although influenza A/H1N1 virus has caused both sporadic illness and epidemics throughout the world, there have been few cases and no outbreaks reported in older persons. Using a surveillance program for detection of viral respiratory tract illness, we documented an outbreak of influenza (A/Brazil/78(H1N1) infection in one floor of a chronic disease hospital. We prospectively studied all 32 patients and 16 personnel on that floor. Infection was proved in 11 subjects by serology and/or virus isolation, including 9 patients (median age, 84 yr) and 2 personnel (36 and 58 yr of age), for attack rates of 28% and 12.5%, respectively. Six patients had fever, 38 degrees C to 39 degrees C, lasting for 1 to 5 days (median, 4), and/or respiratory and constitutional symptoms lasting for 2 to 17 days (median, 13). Bacterial pneumonia occurred in 1 patient 12 days after the onset of upper respiratory tract illness. Risk factors for acquisition of infection in patients included a nonvaccinated state (p = 0.03) and a preinfection antibody titer of less than or equal to 32 (p = 0.02). These findings indicate that older persons are at risk for infection with influenza A/H1N1 virus, which may also cause outbreaks of respiratory illness in the elderly institutionalized population similar to other influenza viruses.