Mortality rates for colorectal cancer in many developed countries have declined in women more than in men. Possible explanations of the sex differentials in colorectal cancer mainly, but not only, refer to different exposure to exogenous hormones. This paper aims to review the available epidemiological evidence on this issue. Seven cohort studies reported information on HRT use and colorectal cancer risk, with relative risks (RRs) around or below unity, and significant inverse association was found in two of them. Of 12 case-control studies, five reported significant risk reductions among ever-users of HRT, while two investigations showed moderate, non-significant inverse associations and none showed a significant increased risk. Two recent meta-analysis showed a 20% reduction in the risk of colon cancer among current users. Overall, the studies reviewed support the existence of an inverse association between colorectal cancer and HRT. Although these epidemiological observations are consistent, surveillance bias may account for part of the association.
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