Although parity is associated with a decreased risk of ovarian cancer in the general population, this association among women with a family history is less clear. We examined this question in a prospective cohort of 31,377 Iowa women 55–69 years of age at baseline. Relative risks (RRs) and 95% confidence intervals (CIs) were estimated through Cox regression. We identified 181 incident epithelial ovarian cancers through 13 years of follow-up. At baseline, 14% of the women reported breast or ovarian cancer in a first-degree relative, and an additional 12% reported a family history in a second-degree relative. Among women without a family history of breast or ovarian cancer in a first-degree relative, nulliparous women were at slightly increased risk of ovarian cancer (RR = 1.4, 95% CI = 0.9–2.4) compared with parous women, whereas among women with a family history, nulliparous women were at a much higher risk (RR = 2.7, 95% CI = 1.1–6.6) than parous women. Similar results were seen when family history included first- or second-degree relatives with breast or ovarian cancer or a first- or second-degree relative with ovarian cancer only. Nulliparity may be more strongly associated with an increased risk of ovarian cancer among women with a family history of breast or ovarian cancer, compared with women who do not have a family history of those cancers.