A serological study was undertaken to determine the prevalence of antibody to Chlamydia trachomatis in women and to investigate any possible role of the organism in infertility and pelvic inflammatory disease. Thirty-seven per cent of pregnant women were found to have antibodies to Chl. trachomatis, as were 69% of women with pelvic inflammatory disease. Eighty-five per cent of women who were infertile due to inflammatory tubal damage and 78% who were infertile secondary to ectopic pregnancy had antibody as compared with 56% of women who were infertile for other reasons. Sperm bank donors and children showed low prevalences (16% and 3%, respectively). Exposure to Chl. trachomatis is widespread in sexually active women and appears to have a role in pelvic inflammatory disease and infertility that is due to inflammatory tubal disease.