Influenza A epidemics were associated with a doubling to tripling of pneumonia rates among adult members of a prepaid medical care group studied between 1963 and 1975. Rates of influenza A associated with pneumonia increased with age. Influenza B epidemics did not have a similar effect. Overall pneumonia rates were highest in children younger than 5 years, but in this age group, other respiratory viruses dominated as causative agents. Influenza A and B epidemics were not always synchronized with those reported for the United States, and rates of influenza A infection varied between urban and suburban areas in sequential epidemics. In 1974, a year practically free from influenza A, a prolonged Mycoplasma pneumoniae epidemic kept rates of pneumonia high, especially during the summer.