Despite major improvements in the diagnosis of pathogenic organisms causing acute respiratory infections (ARIs), details of infections caused by atypical pathogens are not well understood, particularly in developing countries. This clinical and epidemiological research was conducted in Bangladesh to explore the prevalence of atypical pathogens in causing childhood pneumonia. Sixty-four children with ARI were studied at the Pediatric Outpatient Department of Dhaka Medical College Hospital, Bangladesh, during September through December 2000. In addition to clinical examination, hematological, radiological, and bacteriological examinations were performed. Antibody titers from paired sera against Mycoplasma pneumoniae and Legionella spp. in the acute and convalescent phases revealed that none of these children were infected with M. pneumoniae, while only one serum sample was positive for L. pneumophila serogroup 4. Antibody titers against Chlamydophila (Chlamydia) pneumoniae, determined by an indirect microimmunofluorescence method, and by an enzyme-linked immunosorbent assay (ELISA) kit (HITAZYME C. pneumoniae kit) indicated that 13 children (20.3%) were infected with C. pneumoniae. Our results indicate a high prevalence rate of C. pneumoniae, suggesting it is as an important causative pathogen of childhood pneumonia in Bangladesh.