Non-gonococcal ophthalmia neonatorum was the first recognized manifestation of sexually transmitted chlamydial infection and for many years it was thought to be the only manifestation in infants born to infected mothers. In the 20 years since techniques for isolation of Chlamydia trachomatis in cell culture were described, many important effects of chlamydial infection, including afebrile pneumonia in infants, have been established. Prospective studies have determined the incidence of chlamydial infection in pregnant women and the risk of transmission of infection to their infants. They have shown that these are among the commonest perinatal infections. It is estimated that at least 1% of infants in this community have chlamydial conjunctivitis and up to 5/1000 will develop pneumonia. Chlamydial infections are characterized by a subacute onset and, without appropriate treatment, a prolonged course. Although they are rarely fatal, symptoms are sometimes severe and there may be long-term sequelae. The recent development of rapid and relatively inexpensive methods for direct detection of chlamydiae in clinical specimens will facilitate the diagnosis and treatment of infections in infants.