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Chlamydia trachomatis (CT) as a risk factor for ectopic pregnancy (EP) has mainly been established through seroepidemiologic cross-sectional studies.The goal of this study was to obtain EP risk estimates for women diagnosed with CT using women with negative tests as the reference group.We linked prospectively collected CT laboratory data (1990–2003) to EP hospital data (discharge and outpatient registries) in a nested case–control study. Six hundred sixteen women with CT test(s) before first EP were eligible as cases. Three controls were matched to each case for year of birth, age at first test, and number of prior tests.Previous CT infection was associated with elevated EP risk (odds ratio [OR], 1.4; 95% confidence interval [CI], 1.0–2.0). In stratified analysis, the association was only significant for the youngest women (born 1970–1984) who had a nearly complete CT testing history (OR, 2.1; 95% CI, 1.3–3.2).A history of diagnosed CT infection is associated with a 2-fold increased EP risk.