In a retrospective analysis of lower respiratory tract infections in an ex-injection-drug users community, we found an outbreak (April to July 1991) of Chlamydia pneumoniae infection. The epidemic occurred in a group of 26 community members (23 men and 3 women, mean age, 28.9--3 years) living and working together, who underwent acute and convalescent serologic tests for Mycoplasma pneumoniae, Legionella pneumophila, cytomegalovirus, adenovirus, Coxiella burnetii, and Chlamydia pneumoniae. All subjects were submitted to chest radiograph, while sputum and blood cultures were performed in symptomatic patients. Antibodies to C pneumoniae were determined by a microimmunofluorescence test. Among all subjects studied (13 HIV-1 positive and 13 HIV-1 negative), 11 (8 HIV-positive and 3 HIV-negative) developed pneumonia, 2 (1 HIV-positive and 1 HIV-negative) developed pharyngitis, and 2 (1 HIV- positive and 1 HIV-negative) developed flu-like syndromes sustained by C pneumoniae; in 4 subjects (2 HIV-positive and 2 HIV-negative) suffering from flu-like syndrome, no causal agents were found. Seven subjects (one HIV-positive and six HIV- negative) remained asymptomatic without any evidence of infection. The prevalence of antibodies to C pneumoniae in HIV-1-positive subjects observed in a sample of community members was significantly higher than in HIV-1-negative subjects. C pneumoniae seems to be involved in respiratory tract infections in HIV-1-infected subjects. Our data suggest that C pneumoniae should be included in the diagnostic approach of respiratory infections in HIV-infected subjects.