Respiratory tract colonization with Chlamydia trachomatis commonly occurs in natally acquired chlamydial infection and is sometimes associated with a chronic, afebrile pneumonia that has relatively distinctive clinical characteristics. To further define the frequency and clinical characteristics of lower respiratory tract disease associated with C trachomatis, we grouped 56 infants aged less than 6 months with afebrile pneumonia according to nasopharyngeal shedding of Chlamydia and viruses and compared their illnesses. Forty-one (73%) were positive for C trachomatis (23 had C trachomatis only, while 18 had C trachomatis plus a virus [cytomegalovirus, respiratory synctial virus, adenovirus, rhinovirus, or enterovirus]), and 15 were C trachomatis negative (nine had a virus only, and six had neither C trachomatis nor virus). The 41 infants with C trachomatis alone or C trachomatis plus a virus were similar clinically and differed significantly from other infants in several ways: (1) onset of symptoms before 8 weeks of age; (2) gradually worsening symptoms; (3) presentation for care at 4 to 11 weeks of age; (4) presence of conjunctivitis and ear abnormalities; (5) chest roentgenograms showing bilateral, symmetrical, interstitial infiltrates and hyperexpansion; (6) peripheral blood eosinophils greater than or equal to 300/cu mm; and (7) elevated values for serum immunoglobulins M, G, and A. Pediatrics 63:192--197, 1979, Chlamydia trachomatis, pneumonia, afebrile pneumonia, interstitial pneumonia.