Objective Few studies have examined the risk of an ovarian serous borderline tumor (SBT) associated with parity, infertility, oral contraceptives (OCs), or hormone replacement therapy (HRT), which was the study aim. Methods This nationwide case-control study included all women with an SBT diagnosis in Denmark, 1978–2002. SBTs were confirmed by centralized expert pathology review. For each case, 15 age-matched female controls were randomly selected using risk-set sampling. Cases and controls with previous cancer (except for non-melanoma skin cancer) and controls with bilateral oophorectomy or salpingo-oophorectomy were excluded. Conditional logistic regression was used to estimate adjusted odds ratios (ORs) and 95% confidence intervals (CIs). Results We found a strongly decreased risk of SBTs among parous women which decreased with increasing number of children (p�<�0.01). Older age at first birth also decreased the SBT risk (p�=�0.03). An increased SBT risk was associated with infertility (OR�=�3.31; 95% CI: 2.44–4.49), which was present both among parous and nulliparous women. HRT use increased the SBT risk (OR�=�1.32; 95% CI: 1.02–1.72), whereas OC use decreased the risk (OR�=�0.40; 95% CI: 0.26–0.62). Conclusions Our nationwide study with expert histopathologic review of all SBTs showed that parity, infertility, use of HRT, and use of OCs, respectively, were strongly associated with the risk of SBTs. This is the first study to report a strong and significantly decreased SBT risk associated with OC use and a significantly increased risk with infertility, and HRT use. This supports that SBTs and serous ovarian cancer share similar risk factors.