We studied the fertility of the parents of 163 women with epithelial ovarian carcinoma in two hospitals in New York City, compared with the parents of 159 controls from similar neighborhoods. We used unconditional logistic regression to control for covariates, including parity, oral contraceptive use, age at menarche, and Jewish ancestry. Compared with women with zero or one sibling, those with two, three or four or more siblings had adjusted odds ratios (95% confidence intervals) of 0.91 (0.47–1.77), 0.61 (0.28–1.37), and 0.50 (0.26–0.98). On average, each additional sibling was associated with a risk reduction to 0.80 (0.66–0.98). These findings support the hypothesis that heritable conditions associated with reduced fertility of the subjects’ parents may contribute to risk and may explain some of the effects of parity on the risk of this carcinoma. Alternatively, they may reflect some unidentified aspect of the environment experienced by girls growing up in small families. If confirmed in other data sets, these findings imply that sibship size might have confounded previous estimates of risk associated with a family history of cancer or with Jewish ethnicity.