Objective: To examine IVF-ET outcome in patients with and without serologic evidence of Chlamydia trachomatis infection and chlamydia heat shock protein 60 (CHSP 60) antibodies. Design: Retrospective case control. Setting: University-affiliated IVF-ET program. Main Outcome Measures: A total of 195 IVF-ET patients with tubal factor infertility underwent oocyte pick-up, 166 of these women had ET resulting in a total. of 37 pregnancies. Serum antibody tasting for evidence of remote C. trachomatis and CHSP 60, as well as pregnancy outcome, were determined for all patients. Results: There were no differences in pregnancy rates or outcomes between C. trachomatis seropositive versus seronegative groups: 27/118 (23%) C. trachomatis seropositive versus 10/77 (13%) C, trachomatis seronegative patients achieved pregnancy per oocyte pick-up. Pregnancy rates per ET were 27/105 (26%) in C. trachomatis seropositive versus 10/61 (16%) C. trachomatis seronegative patients. In the C. trachomatis positive subgroup, significantly higher pregnancy rates were found in the CHSP 60 antibody positive patients: 24/67 (36%) CHSP 60 positive versus 3/51 (6.0%) CHSP 60 negative patients were pregnant after oocyte pick-up (OR = 8.9, 95% CI = 2.3 to 27.5). Pregnancy rates per ET were 24/57 (42%) in CHSP 60 positive versus 3/48 (7%) CHSP 60 negative patients (OR = 10.9, 95% CI = 2.8 to 33.6). There were no significant differences in any group when examining the following pregnancy outcomes: spontaneous abortion, ectopic pregnancy, preterm and multiple pregnancy rates. Conclusions: [1] There are no differences in pregnancy rates or outcomes in patients with and without serologic evidence of previous C. trachomatis infection. [2] In women seropositive for C. trachomatis, significantly higher pregnancy rates are found in women who are CHSP 60 antibody positive versus negative. [3] Pregnancy outcomes do not appear to be different between these groups.