Objective To determine if Chlamydia trachomatis (C. trachomatis) seropositivity, as detected by the C. trachomatis elementary body (EB)-based enzyme-linked immunosorbent assay [EB ELISA] predicts pregnancy and pregnancy outcome among infertile women with documented tubal patency. Design Cohort study. Setting Outpatient clinics. Patient(s) In all, 1,250 infertile women with documented tubal patency enrolled in 1 of 2 randomized controlled trials: Pregnancy in Polycystic Ovary Syndrome II; and the Assessment of Multiple Intrauterine Gestations From Ovarian Stimulation. Intervention(s) Sera were analyzed for anti-C. trachomatis immunoglobulin G (IgG)1 and IgG3 antibodies, using a research C. trachomatis EB ELISA. The optical density (OD)405 readings of ≥0.35 and ≥0.1 were considered positive for IgG1 and IgG3, respectively. Main Outcome Measure(s) Primary outcomes included pregnancy, live birth, and ectopic pregnancy. Log-linear regression was used to determine the relative risk after adjusting for age, race, treatment medication, smoking status, and current alcohol use. Result(s) A total of 243 (19%) women were seropositive for anti-C. trachomatis IgG3. They tended to be nonwhite and smokers. Anti-C. trachomatis IgG3 seropositive women were significantly less likely to conceive (risk ratio [RR] 0.65, 95% confidence interval [CI] 0.52-0.83) or to have a live birth (RR 0.59, 95% CI 0.43-0.80); these associations were weakened after adjusting for number of hysterosalpingography-documented patent tubes (RR 0.73, 95% CI 0.56-0.97) and (RR 0.73, 95% CI 0.50-1.04), respectively. Anti-C. trachomatis IgG3 seropositive women who conceived had a ×2.7 risk (95% CI 1.40-5.34) of ectopic pregnancy. Conclusion(s) Even in the presence of tubal patency, anti-C. trachomatis IgG3 seropositivity is associated with a lower likelihood of pregnancy. Anti-C. trachomatis IgG3 seropositive women have as high as 3 times the risk of ectopic pregnancy. Clinical Trial Registration Number PPCOSII: NCT00719186 and AMIGOS: NCT01044862.