BACKGROUND: Chlamydial infection is associated with tubal factor infertility (TFI); however, assessment of prior chlamydial infection and TFI is imperfect. We previously evaluated a combination of serological assays for association with TFI. We now describe the chlamydial contribution to TFI using a newer Chlamydia trachomatis Pgp3 enhanced serological (Pgp3) assay. METHODS: In our case-control study of women 19-42 years old with hysterosalpingogram-diagnosed TFI (cases) and non-TFI (controls) in two U.S. infertility clinics, we assessed possible associations and effect modifiers between Pgp3 seropositivity and TFI using adjusted odds ratios (aOR) with 95% confidence intervals (CI) stratified by race. We then estimated the adjusted chlamydia population attributable fraction (aPAF) with 95% CI of TFI. RESULTS: All black (n=107) and 618 of 620 non-black women had Pgp3 results. Pgp3 seropositivity was 25.9% (19.3-33.8%) for non-black cases, 15.2% (12.3-18.7%) for non-black controls, 66.0% (95% CI 51.7-77.8%) for black cases, and 71.7% (59.2-81.5%) for black controls. Among 476 non-black women without endometriosis (n=476), Pgp3 was associated with TFI (aOR 2.6 [1.5-4.4]), adjusting for clinic, age, and income; chlamydia TFI aPAF was 19.8% (95% CI 7.7-32.2%) in these women. Pgp3 positivity was not associated with TFI among non-black women with endometriosis nor among black women (regardless of endometriosis). CONCLUSIONS: Among non-black infertile women without endometriosis in these clinics, 20% of TFI was attributed to chlamydia. Better biomarkers are needed to estimate chlamydia TFI PAF, especially in black women.