It is well known that Chlamydia pneumoniae is an important respiratory pathogen. In this study, the prevalence of specific antibodies to C. pneumoniae in hospitalized patients with community-acquired pneumonia (943 adult females, 990 adult males and 185 children) was evaluated over a period of 7 y (1993-99). Two serum samples were obtained from all of the patients: 1 on admission and the other 3 weeks later. The specimens were tested for C. pneumoniae IgG, IgM and IgA antibodies by means of a microimmunofluorescence test. Acute infection with C. pneumoniae was determined in 9.4% of females and 13.1% of males. In children and adolescent patients, the microimmunofluorescence test showed recent infection in 8.6% of cases, with the highest prevalence occurring in the 11-15 y age group. The highest prevalence of C. pneumoniae pneumonia was found in 1995 and 1999: 15.4% and 13.6% respectively. The results obtained showed that C. pneumoniae is persistently present in the population of Slovenia.