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The preponderance of observational studies describe an association between the use of estrogen alone and a lower incidence of colorectal cancer. In contrast, no difference in the incidence of colorectal cancer was seen in the Women's Health Initiative (WHI) randomized, placebo-controlled trial with estrogen alone after a mean intervention of 7.1 years and cumulative follow-up of 13.2 years. This study extends these findings by providing detailed analyses of the effects of estrogen alone on the histology, grade, and stage of colorectal cancer, relevant subgroups, and deaths from and after colorectal cancer.The WHI study was a randomized, double-blind, placebo-controlled trial involving 10,739 postmenopausal women with prior hysterectomy. Participants were assigned to conjugated equine estrogen at 0.625 mg/d (n = 5279) or a matching placebo (n = 5409). Rates of colorectal cancer diagnoses and deaths from and after colorectal cancer were assessed throughout the study.Colorectal cancer rates in the estrogen-alone and placebo groups were comparable: 0.14% and 0.12% per year, respectively (hazard ratio [HR], 1.13; 95% confidence interval [CI], 0.83-1.58; P = .43). Bowel screening examinations were comparable between the 2 groups throughout the study. The grade, stage, and location of colorectal cancer did not differ between the randomization groups. There were more colorectal cancer deaths in the estrogen-alone group (34 [0.05%] vs 24 [0.03%]; HR, 1.46, 95% CI, 0.86-2.46; P = .16), but the difference was not statistically significant. The colorectal cancer incidence was higher for participants with a history of colon polyp removal in the estrogen-alone group (0.23% vs 0.02%; HR, 13.47; nominal 95% CI, 1.76-103.0; P .001).The use of estrogen alone in postmenopausal women with prior hysterectomy does not influence the incidence of colorectal cancer or deaths from or after colorectal cancer. A possibly higher risk of colorectal cancer in women with prior colon polyp removal who use estrogen alone requires confirmation.