From the article:
The scientists followed 44,241 women for approximately 20 years. Compared to postmenopausal women not using hormone replacement therapy, users of estrogen-only therapy had a 60 percent greater risk of developing ovarian cancer. The risk increased with length of estrogen use. The women, who were followed from 1979 to 1998, were former participants in the Breast Cancer Detection Demonstration Project, a mammography screening program conducted between 1973 and 1980.
“The main finding of our study was that postmenopausal women who used estrogen replacement therapy for 10 or more years were at significantly higher risk of developing ovarian cancer than women who never used hormone replacement therapy,” said James V. Lacey, Jr., Ph.D., lead author of the study from NCI’s Division of Cancer Epidemiology and Genetics.
The relative risk for 10 to 19 years of use was 1.8, which translates to an 80 percent higher risk than non-users, and increased to 3.2 (a 220 percent higher risk than non-users) for women who took estrogen for 20 or more years.
[…]
Two recent large studies found a link between hormone use and ovarian cancer. A large prospective study published last year (JAMA 2001;285:1460-1465) showed that postmenopausal estrogen use for 10 or more years was associated with increased risk of ovarian cancer mortality, and a recent Swedish study (J. Natl. Cancer Inst. 2002;94:497-504) reported that estrogen use alone and estrogen-progestin used sequentially (progestin used on average 10 days/month) may be associated with an increased risk for ovarian cancer. In contrast, estrogen-progestin used continuously (progestin used on average 28 days/month) seemed to confer no increased ovarian cancer risk.