OBJECTIVE: To determine the incidence of Chlamydia pneumoniae respiratory tract infection in HIV-positive or AIDS patients. METHODS: Serum samples from 82 HIV-positive patients with fever and respiratory symptoms were evaluated using microimmunofluorescence assay to detect C. pneumoniae-specific IgG and IgM antibodies. RESULTS: Twenty patients were found to have IgG antibodies to C. pneumoniae at titers ranging between 1:16 and 1:1024. Seven of the patients had evidence of acute C. pneumoniae infection (a fourfold rise in the titer of IgG antibody, or a single IgG titer of greater-than-or-equal1:512, or a single IgM titer greater-than-or-equal1:16). Five were diagnosed as having pneumonia and two bronchitis. No co-infection with other respiratory tract pathogens was found. CONCLUSIONS: Results of this study indicate that C. pneumoniae may play a role in the etiology of respiratory tract infections in HIV-positive patients; this fact should affect empirical antibiotic prescription.