A retrospective serological study was performed on sera from 1982-83 and 1989 to investigate the incidence of Chlamydia pneumoniae infection in hospital treated patients with pneumonia in Orebro County. Paired sera from 231 patients were available and 3 cases (1.3%) of C. pneumoniae infection were documented by microimmunofluorescence. The results differ from those reported by others implying regional differences of C. pneumoniae as causative agent of pneumonia. The findings from Orebro County do not support any change in current antibiotic policy and we still regard penicillin as the first drug of choice in community-acquired pneumonia.