Middle distillate fuels (MDFs), which include jet fuel, kerosene, and diesel fuel, are a class of hydrocarbons distilled from crude oil at approximately 350-700 degrees F (176-371 degrees C). Although MDFs generally do not contain appreciable levels of potentially carcinogenic polycyclic aromatic compounds (PACs), they have produced weak tumorigenic responses in mouse skin characterized by low tumor yield and long latency. Recent studies demonstrated that the tumorigenic effects of these MDFs were dependent upon chronic dermal irritation. In the absence of skin irritation, tumors did not develop. Mechanistic studies suggest that straight-run MDFs containing low levels of PACs cause skin tumors through a nongenotoxic mechanism. MDFs cause chronic skin irritation and injury with repeated application to the skin. They have been found to have little or no activity in the modified Ames mutagenicity assay, lack tumor initiating activity, and are active skin tumor promoters. It has been hypothesized that the tumorigenic response to MDFs results from the promotion of preexisting, spontaneously initiated cells. Two recent studies, a one-year tumor promotion study and a two-year skin painting study, evaluated the role of skin irritation on the tumorigenic activity of MDFs in mice. MDFs were applied in pure and diluted forms to assess the effect of equal weekly doses of irritating and nonirritating test materials. The tumorigenicity of straight-run MDFs correlated to the level of skin irritation. No significant increase in tumor incidence occurred under conditions that resulted in minimal skin irritation and injury. These studies indicate that the tumorigenic activity of MDFs containing low levels of PACs is secondary to chronic skin irritation. These materials should not present a carcinogenic hazard in the absence of prolonged skin irritation.