Antibodies toChlamydia trachomatis were detected in 54 (47.4%) of 114 infertile women attending Bourn Hall Clinic. Antibodies were solely of the IgG class and mainly of a low titer, suggesting past infection. Antibodies were found in significantly more patients with tubal damage (54.4%) than in women whose infertility was due to other causes (16.6%). Seventy-two women completed in vitro fertilization, with 52 having three embryos replaced. We found that this treatment offered the optimum chance of a pregnancy being established, and 20 (38.5%) of these women became pregnant. Antibodies toC. trachomatic were present in only six (30.0%) of the women becoming pregnant, whereas antibodies were found in 21 (65.6%) of those who failed to become pregnant. Thus past infection withC. trachomatis halved the success rate of in vitro fertilization in these patients. The implications of these findings are relevant to all aspects of infertility from prevention to in vitro fertilization treatment.