The practice of medicine-both past and present-often involves the prescription of specific foods (almost always plants) or their potent derivatives, to treat a wide spectrum of illnesses. Foods that have been ascribed healing properties include the Cruciferae, the allium family, celery, cucumber, endive, parsley, radish and legumes. Review of the epidemiological data, including both cohort and case-control studies, of all cancer sites strongly suggests that plant foods also have preventive potential and that consumption of the following groups and types of vegetables and fruits is lower in those who subsequently develop cancer: raw and fresh vegetables, leafy green vegetables, Cruciferae, carrots, broccoli, cabbage, lettuce, and raw and fresh fruit (including tomatoes and citrus fruit). Other data suggest that foods high in phytoestrogens, particularly soy (which contains isoflavones), or high in precursor compounds that can be metabolized by gut bacteria into active agents, particularly some grains and vegetables with woody stems (which contain precursors to lignans), are plausibly associated with a lower risk of sex-hormone-related cancers. The human evidence for these latter associations is not strong. There are many biologically plausible reasons why consumption of plant foods might slow or prevent the appearance of cancer. These include the presence in plant foods of such potentially anticarcinogenic substances as carotenoids, vitamin C, vitamin E, selenium, dietary fibre (and its components), dithiolthiones, isothiocyanates, indoles, phenols, protease inhibitors, allium compounds, plant sterols, and limonene. Phytoestrogens are also derived from some vegetables and berries as well as grains and seeds. Most of the data for the observations on the anticarcinogenic potential of all of these compounds have come from animal and in vitro studies. At almost every one of the stages of the cancer process, identified phytochemicals are known to be able to alter the likelihood of carcinogenesis-occasionally in a way that enhances risk but usually in a favourable direction. For example, glucosinolates and indoles, thiocyanates and isothiocyanates, phenols, and coumarins can induce a multiplicity of phase II (solubilizing and usually inactivating) enzymes; ascorbate and phenols block the formation of carcinogens such as nitrosamines; flavonoids and carotenoids act as antioxidants, essentially disabling the carcinogenic potential of specific compounds; lipid-soluble compounds such as carotenoids and sterols may alter membrane structure or integrity; some sulphur-containing compounds suppress DNA and protein synthesis; carotenoids can suppress DNA synthesis and enhance differentiation; and phytoestrogens compete with estradiol for estrogen receptors in a way that is generally antiproliferative. Consumption of diets low in plant foods results in a reduced intake of a wide variety of those substances that can plausibly lower cancer risk. In the presence of a diet and lifestyle high in potential carcinogens (whether derived from fungal contamination, cooking or tobacco) or high in promoters (such as salt and alcohol), overall risk of cancer at many epithelial sites is elevated. Plant foods appear to exert a general risk-lowering effect; the patterns of exposure to cancer initiators and promoters and of genetic susceptibility may determine the variations in the site-specific risks of cancer seen across populations.