The available literature on estrogen metabolism and estrogen metabolites involved in carcinogenesis is reviewed. Endogenous estradiol metabolism leads to metabolic products that can have various, and, to some extent, contrary, biologic effects. Thus, there are numerous research findings on the stimulation and inhibition of cancer growth by estrogen metabolites. Furthermore, there are indications that, in certain types of cancer, the production of growth-stimulating estradiol metabolites is increased. There are also reports on substances that can influence estradiol metabolism. So far, only a few estradiol metabolites have been examined with respect to their influence on the development and growth of cancer. It is presumed that other metabolites can also intervene directly or indirectly in the cancer process, but there is a great lack of research in this area. An understanding of the actions of estradiol metabolites may open up new avenues for the therapy of malignant diseases. Although little is known about the biologic effects of most of the estradiol metabolites, the reported actions of certain estradiol metabolites already justify clinical investigations on their possible beneficial uses in tumor therapy.