Conjunctival and nasopharyngeal samples were collected from 51 infants aged 1 month-1 year, who have been hospitalized for lower respiratory tract disease which had begun on the 4-12th week of life and has shown recurrences since then. Acute period blood samples were available from 42 of these children. Chlamydia trachomatis antigen was searched for in the conjunctival and nasopharyngeal specimens by a direct fluorescence test (MikroTrak, Syva Co.), and serum anti-chlamydial IgM antibodies were detected by enzyme immunoassay (Sero ELISA Chlamydia, Savyon Diagnostics). Chlamydia trachomatis was detected from at least one site in 20 children, and 15 children were positive for anti-chlamydial IgM antibodies. Serum samples were available in 16 of 20 children who were positive for C. trachomatis antigens, and 12 had IgM antibodies. Three infants who were antigen negative, also had anti-chlamydial antibodies. Whether antigen positive or not, all patients with antichlamydial antibodies were considered to have pneumoniae due to C. trachomatis, and those who had only antigens without any antibodies, to be superficially infected and carry the organism symptomatically or asymptomatically.