Diffuse carcinogenic risks, that is, those of low potency involving large areas of population and sometimes all mankind, pose a serious public health problem. Controlling these risks might help to reduce the incidence of, and mortality from, cancer. Because of their low expected carcinogenic potential, these risks are difficult to expose or assess. Epidemiologic investigation is of limited use in this field and yields its data too late to be useful. Experimental studies offer the only possible approach for assessing such risks. To increase experimental sensitivity and consistency of results, mega-experiments must be designed. That is, experiments that use a large number of animals with a well-known basic tumorigram, that extend the exposure and the biophase for as long as possible, that carefully observe the effects, and that are performed with suitable standardized methods. In the last 15 years the Ramazzini Foundation, in its Cancer Research Center at Bentivoglio, has conducted or planned five mega-experiments. Initial results indicate the great potential of these methods for identifying and assessing diffuse risks.