Context: The association between menopausal hormone replacement therapy and ovarian cancer is unclear.
Objective: To determine whether hormone replacement therapy using estrogen only, estrogen-progestin only, or both estrogen only and estrogen-progestin increases ovarian cancer risk.
Design: A 1979-1998 cohort study of former participants in the Breast Cancer Detection Demonstration Project, a nationwide breast cancer screening program.
Setting: Twenty-nine US clinical centers.
Participants: A total of 44 241 postmenopausal women (mean age at start of follow-up, 56.6 years).
Main outcome measure: Incident ovarian cancer.
Results: We identified 329 women who developed ovarian cancer during follow-up. In time-dependent analyses adjusted for age, menopause type, and oral contraceptive use, ever use of estrogen only was significantly associated with ovarian cancer (rate ratio [RR], 1.6; 95% confidence interval [CI], 1.2-2.0). Increasing duration of estrogen-only use was significantly associated with ovarian cancer: RRs for 10 to 19 years and 20 or more years were 1.8 (95% CI, 1.1-3.0) and 3.2 (95% CI, 1.7-5.7), respectively (P value for trend <.001), and we observed a 7% (95% CI, 2%-13%) increase in RR per year of use. We observed significantly elevated RRs with increasing duration of estrogen-only use across all strata of other ovarian cancer risk factors, including women with hysterectomy. The RR for estrogen-progestin use after prior estrogen-only use was 1.5 (95% CI, 0.91-2.4), but the RR for estrogen-progestin-only use was 1.1 (95% CI, 0.64-1.7). The RRs for less than 2 years and 2 or more years of estrogen-progestin-only use were 1.6 (95% CI, 0.78-3.3) and 0.80 (95% CI, 0.35-1.8), respectively, and there was no evidence of a duration response (P value for trend =.30).
Conclusion: Women who used estrogen-only replacement therapy, particularly for 10 or more years, were at significantly increased risk of ovarian cancer in this study. Women who used short-term estrogen-progestin-only replacement therapy were not at increased risk, but risk associated with short-term and longer-term estrogen-progestin replacement therapy warrants further investigation.