In vivo and in vitro studies; epidemiological and statistical publications concerning a possible carcinogenic effect of pro­gesterone - and progestins - raise a question mark. Too many contradictions exist in experimental and clinical studies. A very small increased risk of breast cancer with estrogen therapy has been shown; however bias are often present. It is not clearly demonstrated that the adjunction of progestin to estrogen significantly increases the risk, nor that it brings about a protective effect. Some recent works seem to indicate that continuous estroprogestin replacement therapy might be deleterious, while a sequential regimen would be not. This is to be confirmed. The results of the Women's Health Initiative Study have been analyzed. The data and conclusions of the medical literature should be interpreted very cautiously.