Approximately 20% of all deaths in the United States are due to cancer. Cancers of the hormonal tissues such as breast, uterus, ovary in women and prostate in men account for about 8% and 5% of total mortality and 30% and 11% of cancer mortality in women and men, respectively. Diet is considered to be a major and important environmental factor contributing to cancers of hormonal tissues. Breast, uterus, and ovary cancers in women and prostate cancers in men were positively correlated with high fat consumption, high body weight (body mass), body fat, and obesity. A major mechanism for development of these cancers appears to be mediated through increased levels of hormones, especially estrogens. Adipose tissue is considered to be one of the major sources of extraglandular estrogen, produced by aromatization of androgen precursors. Weight reduction decreases the estrogen levels possibly due to a decrease in body fat, thus decreasing the risk for cancers of the hormonal tissues. Dietary fiber may modify the risk for these cancers by influencing estrogen metabolism, recirculation, and excretion. Vitamin A and its precursors may decrease the risk for prostate cancer. Iodine deficiency may increase the risk for thyroid neoplasms in humans and experimental animals. Tumors of the hormonal tissues are the most common tumors in laboratory rodents, especially rats and mice. Incidences of mammary and anterior pituitary tumors had significant and positive correlation with body weight in rats and mice. Lowering the body weight by either decreased caloric intake or other means (e.g., exercise, increased fiber consumption) markedly lowered the incidences of these tumors in laboratory rodents. Laboratory studies indicated that mammary tumor rates in rats may not depend on the amount of fat consumed per day. The mammary tumor-promoting effect of fat may be due to complex interactions involving energy intake and energy retention (body mass) mediated through paracrine, endocrine, and neurohormonal mechanisms. Dietary protein may influence chemically induced tumors by affecting the metabolism of chemicals through enzyme induction. Thus, environmental factors such as diet are considered to be major and important factors for tumors of the hormonal tissues such as breast, uterus, and ovary in women and prostate in men. Diet and associated body weight are considered to be the major factors for tumors of hormonal tissues such as mammary and pituitary glands in rodents, especially rats. Modification of diet and a decrease in caloric intake may markedly decrease the incidence or delay the development of tumors of hormonal tissues in humans and in experimental animals.