Relatively little is known about the mechanisms underlying carcinogenesis in humans. Caloric restriction strongly inhibits the development of neoplasia in rodents, and there is evidence of a positive relationship between cancer and body weight in humans. Caloric restriction early in life is also known to permanently diminish organ cellularity. A recent link between adult stature and cancer incidence similarly implicates a lasting effect for growth and possibly for early nutrition in carcinogenesis. It is postulated that cancer risk is proportional to the number of proliferating cells, which in turn depends on both the number of cells and the rate of cell division within the tissue. This hypothesis is consistent with several aspects of human carcinogenesis, including multistage models and the epithelial origin of most cancers.