Pneumonia due to Mycoplasma pneumoniae was monitored in a large prepaid medical-care group in Seattle, Washington, between 1963 and 1975. The disease was diagnosed by isolation of M. pneumoniae and/or significant rises in titer of complement-fixing (antilipid) antibody in paired sera. Infection was endemic without significant seasonal fluctuations. Two epidemics occurred: the first peaked in January 1967, the second late in the summer of 1974. Total rates of pneumonia infection in children increased during M. pneumoniae epidemics, but epidemics of infection with respiratory syncytial virus had a greater effect. Age-specific attack rates for M. pneumoniae pneumonia among children aged five to nine years (about six per 1,000) were about twice the rates for younger children and four times those for adults. Serologic study of healthy schoolchildren showed annual rates of infection that paralleled but greatly exceeded rates of recognized M. pneumoniae pneumonia. Infection rates varied from 2% in endemic years to 35% in epidemic periods. A higher proportion of infections among children aged five to nine years than among adolescents aged 15-19 years resulted in pneumonia.