We have reviewed the literature regarding the food sources, potency, population intakes, and known biological effects of phytoestrogens in humans using MEDLINE data base from the years 1975-1996. Over 600 articles pertinent to the metabolism of phytoestrogens, including female reproduction (in particular menstruation and menopause), cardiovascular disease, osteoporosis, and cancer were assessed including relevant case control or cohort studies, as well as randomized trials and review articles. Epidemiological studies regarding human data were included, as well as human cell line and animal studies when there were no relevant human data available. We conclude that phytoestrogens exhibit physiological effects in humans. Mild estrogenic changes occur in postmenopausal women. Benefits are seen regarding hypercholesterolaemia. Epidemiological, animal, and in vitro data encourage further assessment of the role of phytoestrogens in cancer prevention.