The environment is becoming an increasingly important factor in the aetiology of cancer. In addition to physical carcinogenic substances, chemical agents, in particular, are identified in ever increasing numbers as being oncongenic. This has been experimentally shown in the case of more than 1000 substances, of which 30 are also regarded as carcinogenic in humans. To be effective, aetiological research should lead to the elimination of the substances which are identified as harmful. In the present state of our knowledge, this will be a considerable contribution to prophylaxis.
