For the past two decades, epidemiologists have observed lower risks of lung, breast, prostate, colon, and other cancers in populations that frequently consume fruits and vegetables. Numerous phytoestrogens have been shown to be anticarcinogenic under experimental conditions and may account for at least part of the cancer-prevention effects of fruit and vegetable consumption. These plant constituents include isoflavonoids, coumestans, lignans, phytosterols, and flavonoids. DietSys, the nutrient analysis program associated with the National Cancer Institute Health Habits and History Questionnaire (HHHQ), and other nationally available nutrient analysis databases do not fully assess these constituents. Therefore, we modified DietSys to include these components in foods on the basis of published values. In addition, as part of an epidemiological study of prostate cancer, we modified the food-frequency component of the HHHQ to include the main foods contributing to phytoestrogen intake. Although there are limitations to the consistency and quality of many of the values because they were gathered from a variety of sources, our approach should provide a useful first tool for assessing the epidemiological association between phytoestrogen consumption and cancer risk. Furthermore, this work has already facilitated the identification of the major dietary contributors with phytoestrogen activity and prioritized future laboratory analyses of specific foods toward the development of a more complete and accurate database.