Approximately 20,000 women are diagnosed with ovarian cancer in the United States each year, and some 12,000 women die because of it. Epithelial ovarian cancer, the most common histopathologic type, is uncommon before age 40 years, after which incidence rates increase steeply until age 70-79 years and then decrease somewhat. In the United States, the lifetime risk from birth to age 85 years is about 1.5%. There is general agreement that residence in North America or northern Europe, nulliparity, and having a mother or sister with ovarian cancer are associated with an elevated risk, and that increasing number of pregnancies (whether or not full term), increasing length of oral contraceptive use, and increasing duration of lactation are protective. A history of breast or endometrial cancer appears to be associated with a slight elevation in risk. Apart from oral contraceptive use, none of these characteristics can be modified easily to reduce ovarian cancer risk. However, long-term oral contraceptive use before the menopause could prevent as much as half of all ovarian cancer. At present, the subgroup of the population at highest risk consists of women with a mother or sister with the disease; the lifetime ovarian cancer risk in these women is about 9%. A small fraction of them have families with multiple cases of ovarian cancer and early-onset breast cancer, due largely or entirely to mutated alleles of the gene BRCA1. These women, who have a lifetime risk of breast or ovarian cancer of 85-100%, need aggressive screening and possibly prophylactic surgery.