Chlamydia pneumoniae (TWAR) is a relatively newly discovered respiratory tract pathogen which was first isolated in Taiwan. In order to describe the seroepidemiology of C. pneumoniae in Taiwan, we evaluated 1,085 stored serum samples: 904 from patients, 97 from umbilical cord blood samples, and 84 from medical personnel at the Veterans General Hospital-Kaohsiung, between January 1 and April 30, 1991. Antibodies were determined by the use of a microimmunofluorescence test using elementary bodies of C. pneumoniae AR-39 as antigen. Sera were tested with screening titres of 16 and 512 for immunoglobulin G antibody. The antibody prevalence was found to be 23.1% in young children (6 months to 10 years old), rising to 66.7% in teenagers, and to 96.2% in older age groups. These rates were higher than any reported earlier. The progressively increasing rates of seropositivity found in older individuals indicated a surge of reinfection in these age groups. Only 5 cases were found with micro-IF IgG titres equal to or greater than 512. All were asymptomatic according to the hospital records. In addition to a high prevalence rate in Taiwan, the data also showed high infection rates in teenagers and elderly people.