A group of 205 women with endometrial carcinoma was matched for age, parity, and year of operation with a group of 205 women who had had hysterectomies for benign disease. In the former group, 32 patients had used conjugated estrogens, while in the latter group 12 had used this hormone, yielding a relative risk of 3.1 (P = 0.0008). Users of other forms of systemic estrogens showed similar elevations in relative risk. Relative risk was related to duration of use, progressing from no evidence of risk among those using the hormone for less than 5 years to an 11.5-fold greater risk for those using it for 10 years or more. Risk was also related to the strength of the medication. The relative risk for users of the 1.25-mg tablets was 12.7 as compared to a two- to fourfold greater risk among users of lesser strength tablets.