Postmenopausal estrogen use is associated with increased risk of hormone-related cancers. Endometrial cancer incidence increases rapidly with use of unopposed estrogen1,2; breast cancer incidence, however, increases only after long duration estrogen use.3 Although there is evidence that pituitary and/or sex hormones play an important etiologic role in ovarian cancer,4-6 epidemiologic studies of the association between postmenopausal estrogen use and ovarian cancer have had inconsistent results. Earlier case-control studies report decreased risk,7,8 no association,9-15 or increased risk.5,16-19 More recent and larger case-control studies have suggested increased risk,20-23 particularly with long duration of estrogen use.24-26 However, even the largest previous studies have had limited statistical power to assess the risk associated with long duration of estrogen use.