Chlamydia pneumoniae is a common cause of pneumonia worldwide. This study examined the role of C. pneumoniae in 229 patients with community-acquired pneumonia. The ages of the patients ranged from 2 to 95 years. Sera were assayed for IgM and IgG antibodies with microimmunofluorescence (MIF). An IgM titer equal to or greater than 1:20 and IgG titer equal to or greater than 1:64 were considered positive. The prevalence of positive IgG among all of the patients was 75.1% (172/229). The seroprevalence was 81.8% (9/11) for patients in the 11- to 20-year-old group, 63.6% (14/22) in the 31- to 40-year-old group and 88.1% (52/59) in the 71- to 80-year-old group. All patients had a negative result for IgM antibody. Twenty-five of the patients had an IgG titer equal to or greater than 1:512, indicating the presence of current infection or reinfection. Age older than 60 years (vs. < or = 60 years) was a risk factor for C. pneumoniae seropositivity in patients with community-acquired pneumonia (p < 0.001). Males had a significantly higher seroprevalence of C. pneumoniae infection (p = 0.1010). Patients older than 60 years were more likely to have C. pneumoniae infection (p = 0.1107). In this series, C. pneumoniae infection accounted for 10.9% (25/229) of community-acquired pneumonia. The most common clinical manifestations included fever (92%), productive cough (52%), white blood cell more than 10,000/mm3 (56%), and bilateral pulmonary infiltrate (60%).