We determined the age-related prevalence of complement-fixing antibody to Mycoplasma pneumoniae from the computerized laboratory results of our routine diagnostic department. The material consisted of about 58,000 sera from an 8-year period, 1971 to 1978. Among children less than 1 month old, the frequency of complement-fixing antibody of titers greater than or equal to 8 was low (23%), and no decrease representing loss of maternal antibody was seen thereafter. An unexpectedly early increase in the antibody prevalence up to 62% was seen by 6 months of age. The frequency of antibody was high among young children from the age of 4 months, in whom symptomatic infection due to M. pneumoniae is rare. The frequency of higher titers (greater than or equal to 32) and the geometric mean titer increased more slowly, coinciding with the known age distribution of symptomatic M. pneumoniae disease; the frequency of high titers and the geometric mean titer peaked at the age of 7 to 10 years. Two explanations for the high frequency of complement-fixing antibody to M. pneumoniae in young children are discussed. It may be due to an asymptomatic infection caused by M. pneumoniae or to a nonspecific stimulus by lipids of other organisms, plants, or tissues leading to production of antibodies crossreacting with M. pneumoniae, or it may be due to both of the above. During the study, two extensive epidemics due to M. pneumoniae occurred in Finland, but the prevalence of complement-fixing antibody bore no correlation with these peaks of occurrence.