Overnutrition, as a factor in carcinogenesis, has been a matter of concern for over 80 years. Overnutrition relates to excess intake of calories, and fat is the major contributor to caloric burden. Thus, fat has been the focus of many epidemiological studies, but as long ago as 1975 some investigators were suggesting that excess energy intake might be the major factor relating to cancer incidence. Ecological studies support the idea that a high fat (high energy?) diet may represent a risk for cancer but case-control or follow-up studies generally do not. The effects of undernutrition have been studied experimentally. Mostly conducted in rats or mice, they show virtually uniformly that caloric (energy) restriction inhibits the growth of spontaneous, transplanted or induced tumours. The effect is observed even when the calorie-restricted animals ingest more fat than do the controls. Energy utilization via exercise reduces tumour growth in rats and a life history of physical labour reduces risk in man. The mechanism(s) by which caloric restriction exerts its effects are moot, but it has been shown to reduce insulin levels and to reduce oncogene expression. Energy restriction also increases activity of antioxidant enzymes and leads to enhanced DNA repair. Increased energy flux (by means of decreased intake or increased output) may provide a simple and inexpensive approach to reducing the risk of cancer in man.