Recent studies of the effects of dietary fat on experimental carcinogenesis have probed the influence of the dietary fat source, the time during carcinogenesis when a high-fat diet is fed, the interactions between fat and calories, and the potential mechanisms for the observed effects of fat on cancer. The essential fatty acids in the fat, the degree of their unsaturation, and the location of the unsaturation are all important in determining the influence of a dietary fat source on carcinogenesis. In addition, alterations in dietary fat can modify various sites and forms of cancer differently. Dietary fat has been clearly shown to be important in the promotion of cancer, and recent studies indicate that dietary fat can also be a factor in the initiation of cancer. Although the calories contributed by a high-fat diet also factor into cancer enhancement, dietary fat appears to contribute more than its high-caloric density. One potential mechanism for the influence of dietary fat on carcinogenesis is through modulating the activation of protein kinase C.