Numerous epidemiological studies suggest that diets rich in phytoestrogens (PE), particularly soy and unrefined grain products, may be associated with low risk of some cancers, especially steroid hormone-dependent, e.g. breast and prostate cancers. Epidemiological, in vitro, animal and human studies have investigated the mechanisms involved in PE biological actions, including steroid hormone activity, effects on cell growth, antioxidant activities, inhibition of chemical carcinogenesis and influences on modulators of cancer risk. The question of whether PE may be used as an anticancer therapeutic and/or chemopreventive agents remains unanswered. Clearly, much more information is required, especially concerning the safety of their use. It seems extremely difficult to predict the effects of various PE mixtures present in different human diets. Long-term studies (in vitro, animal, clinical and epidemiological) with well standardized PE preparations are necessary to assess the potential beneficial and adverse effects. With our current state of knowledge, we cannot conclude whether consumption of soy, SIF (soy isoflavones)-supplemented food or the use of particular isoflavones as therapeutics will have positive, null or even adverse effects on cancer (particularly, steroid hormone-dependent) risk and treatment. A brief review of the effects (preventive, antitumor as well as carcinogenic and tumor-stimulating) of PE on various tumor types is presented.