It is the totality of the evidence that links a high level of dietary fat to risk for breast and colon cancers. The evidence is built on descriptive epidemiology, correlation studies, migrant studies, time trends, case-control studies, metabolic epidemiology, experimental animal studies, and biological plausibility. The effects of total caloric intake or of obesity are not as relevant as the specific types and amounts of fat consumed. The effects of fiber in modulating colon cancer risk are inconsistent. This may be due, in part, to the varying effects of differing fibers. The key questions are, which fats and which fibers, and what amount of each, are of etiological and preventive significance?