We studied 52 adult patients admitted to the hospital over an 11-month period with symptoms suggesting pulmonary infection. Twenty-nine had pneumonia and none were receiving immunosuppressive drug therapy. We found definite serological evidence of recent chlamydial infection in 21% (4/19) of patients with pneumonia of unclear etiology, and in 0% (0/33) of patients with other pulmonary conditions. An additional three patients with pneumonia had suggestive serological evidence of recent chlamydial infection. None of the seven patients had culture evidence of bacterial infection, or serological evidence of mycoplasmal, viral, or Legionella infection; none had physical examination findings suggesting intercurrent chlamydial infection of the eye or genitourinary tract. Chlamydia trachomatis may have been a cause of community-acquired pneumonia in some of these patients.