BACKGROUND: Phytoestrogens have been thought to have favorable effects on women's health and perhaps in offsetting cancers. The possible adverse effects of phytoestrogens have not been evaluated. CASES: Abnormal uterine bleeding with endometrial pathology in three women was found to be related to a high intake of soy products. The first woman had postmenopausal bleeding with uterine polyp, proliferative endometrium and a growing leiomyoma. The second woman presented with severe dysmenorrhea, abnormal uterine bleeding, endometriosis and uterine leiomyoma not responding to treatment. The third woman with severe dysmenorrhea, abnormal uterine bleeding, endometriosis and uterine leiomyomata presented with secondary infertility. All three women improved after withdrawal of soy from their diet. CONCLUSION: Additional information on phytoestrogens is necessary to ascertain their safety before they can be routinely used as supplements.