Objective: To assess the risk of borderline ovarian cancer among infertile women treated with fertility drugs. Design: Case-control study.

Setting: Nationwide data obtained from public registers and postal questionnaires. Patient(s): All Danish women,60 years old with borderline ovarian cancer during the period 1989–1994 and randomly selected population controls. The analysis included 231 cases and 1,721 controls. Intervention(s): None. Main Outcome Measure(s): Influence of parity, infertility, and fertility drugs on the risk of borderline ovarian cancer after multivariate confounder control. Result(s): The odds ratio (OR) for borderline ovarian cancer among infertile untreated nulliparous women compared with fertile nulliparous women was 1.9. The OR for borderline ovarian cancer among treated nulliparous women compared with untreated infertile nulliparous women was 1.5, and the OR among treated parous women compared with untreated infertile parous women was 1.5. Conclusion(s): Among fertile women, the difference in the risk of borderline ovarian cancer between nulliparous women and parous women was not statistically significant. Nulliparous women who were infertile and who did not receive medical treatment had a twofold higher risk of borderline ovarian cancer than fertile nulliparous women. There was no statistically significant increase in the risk of borderline ovarian cancer among nulliparous women who were treated with fertility drugs compared with nulliparous untreated infertil