Exogenous estrogen and endometrial carcinoma: case-control and incidence study.

Recent independent case-control studies have indicated that treatment with exogenous estrogen is associated with an increase in the risk of endometrial cancer. This question was studied in Olmsted County, Minnesota, by identifying all cases of endometrial cancer among residents over a 30 year period (1945 through 1974) and by matching four controls to each of the 145 patients. The rate of past exposure to any exogenous estrogen for any duration was about the same for patients as for controls. However, the estimated relative risk of endometrial cancer associated with conjugated estrogen treatment of 6 months or longer was 4.9 (P less than 0.01), and this increased to 7.9 (P less than 0.01) with exposure for 3 years or longer. The risk increased with larger doses (1.25 mg. or more) and continuous administration of conjugated estrogens. The incidence of endometrial carcinoma over the three decades of the study, corrected for an increasing rate of hysterectomy, was constant. Thus to this time, the low rate of use of conjugated estrogens in this region over the past 30 years apparently has not had an appreciable impact on the incidence of endometrial cancer.