Studies of occupational cancer are particularly significant for primary prevention of cancer. Firstly, most cancers, once identified, can be prevented by reasonably simple means, without impinging on personal freedom. Secondly, the prevention of occupational cancers represents a saving of lives and the elimination of illness during the most active period of the lifespan. A third reason for investigating occupational cancers is that, in some cases, workers represent a particularly heavily exposed subgroup, and the occupational setting thus serves as a laboratory for the general environment. The protection of human health in the workplace should not depend solely on the epidemiological demonstration of existing risks; it is essential that results of predictive toxicology from long- and short-term laboratory studies also be used in the identification of possible carcinogenic risk factors.