• Serum samples from 184 Infants and children whose blood was drawn during a clinic visit were tested for antibody to Chlamydia trachomatis, Epstein-Barr virus, and cytomegalovirus. Lifetime illness history was obtained from clinic records. Fifteen percent had anti–C trachomatis IgM antibody. Anti–C trachomatis IgM without IgG was significantly associated with upper respiratory tract syndromes within the 14 days prior to phlebotomy in 6- to 10-year-old patients. This association was not due to polyclonal activation from Epstein-Barr virus infection. A definitive study of chlamydial illness in children rather than infants appears to be indicated.