Chlamydia pneumonia IgG antibody prevalence was examined in a representative sample (2342 subjects) of the population aged 25-59 years in south-western and eastern Finland in 1982 and 1987. Microimmunofluorescence was used to measure IgG antibodies. Prevalence of C. pneumoniae was modelled using the GLIM statistical package assuming that the prevalence had a binomial distribution. The prevalence was assumed to be a function of the year, subjects' gender, age and place of residence. The effect of the year on the prevalence was significant in both regions (P < 0.05 in the south-west and P < 0.001 in the east). In each year the prevalence was higher in the south-west than in the east and it increased between 1982 and 1987 from 55% to 63% in the south-west and from 41% to 59% in the east. The prevalence increased with age, but the age-related antibody pattern was different between sexes. The highest prevalence was found in men aged 49-59 years. The different increase in prevalence with age in the south-western and the eastern parts of the country indicates that the outbreaks of infection were not related. The endemic level of C. pneumoniae infection in the south-west is consistently higher than in the east, or there was an outbreak of C. pneumoniae infection in both the years we examined. Thus it is more likely that the high prevalence in south-western Finland was connected more with the epidemics reported elsewhere in Scandinavia in the same years than with the occurrence of C. pneumoniae infection in eastern Finland.