The etiology of community-acquired pneumonia during a Chlamydia pneumoniae epidemic was studied among 125 hospitalized patients. Etiologic investigations included blood and sputum cultures, pneumococcal antigen detection, and serologic investigations for common respiratory viruses and for Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Mycoplasma pneumococcal antigen detection, and serologic investigations for common and for Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Mycoplasma pneumoniae, and Chlamydia species. A specific microbial agent was identified in 110 patients (88%). Some 48% of the patients had mixed infections. S. pneumoniae was the most common pathogen (55%), followed by C. pneumoniae (43%). C. pneumoniae was found both as a single etiologic agent and as a mixed infection, most often with S. pneumoniae. In conclusion, S. pneumoniae is the most common cause of community-acquired pneumonia, even during a C. pneumoniae epidemic. The organism, alone or with other pathogens (especially S. pneumoniae), may cause community-acquired pneumonia that requires hospital treatment.