OBJECTIVE: To assess the risk of ovarian cancer in women with first-degree relatives with cancer at one of the four most frequent hereditary sites based on validated cancer diagnoses and to examine the association according to age at diagnosis of ovarian cancer and histology. DESIGN: Case-control study. SETTING AND POPULATION: First-degree relatives of 554 women with invasive epithelial ovarian cancer and 1,564 controls were included. METHODS: Analyses were performed using multiple logistic regression models. RESULTS: Ovarian cancer in a first-degree relative was significantly associated with increased risk of ovarian cancer (OR, 2.4; 95% CI, 1.4-4.1 (mother or sister)). Ovarian cancer in a first-degree relative appeared to be a stronger risk factor for early-onset (< or =50 years) ovarian cancer than late-onset (OR, 5.3; 95% CI, 2.0-14.1 vs. OR, 1.8; 95% CI, 1.0-3.4). The positive association with ovarian cancer family history tended to be with non-mucinous tumors. Breast cancer in one first-degree female relative was not significantly associated with risk of ovarian cancer. CONCLUSION: Ovarian cancer in a first-degree relative is a very strong predictor of epithelial ovarian cancer, especially early-onset ovarian cancer. Ovarian cancer in first-degree relatives seems to be positively associated with especially non-mucinous tumors.