Chlamydia trachomatis is the commonest bacterial sexually transmitted disease in the western world. It may be symptomatic or asymptomatic, but a proportion of women subsequently develop pelvic inflammatory disease, with infertility and ectopic pregnancy as potential complications. Previous surveys have suggested that the incidence of infection is highest in younger women and the SIGN guidelines suggest that screening should be carried out opportunistically in all women under 25 years who are sexually active. In pregnancy, chlamydial infection may be associated with premature delivery and perinatal death, but the evidence for this is not conclusive. However, a more definite risk to the neonate is the possible development of chlamydial conjunctivitis or chlamydial pneumonia.