To understand the epidemiology of Chlamydia pneumoniae acute infections in Taiwan, we collected 116 paired and 244 single sera from patients suspected of C pneumoniae infection and conducted microinmunofluorescence test. Eighty-three patients (83/360, 23%) met the diagnostic criteria of current C pneumoniae infection. The C. pneumoniae infections were significantly higher in men than in women (P <= 0.0001) and were most frequent in the group of 40-49 year-olds, and the people older than 70 years old. C pneumoniae infection often occurred in the late autumn lasting to the cold winter and in the transition period between the spring and summer.