Although there is widespread agreement that many cancers have environmental causes we are often unable to see associations between specific cancers and exposure to environmental chemicals. One might also speculate that the more widespread, common-place and 'normal' a chemical exposure is perceived to be then the less likely it will be that the exposure is recognised, let alone be considered to cause cancer. Widespread contamination of air by chemicals associated with internal combustion may be an example of one such 'invisible' carcinogenic exposure. Yet evidence is available which suggests that many leukaemia and lymphoma cases, as well as other cancers, may be caused by this mundane and ubiquitous environmental contamination. The hypothesis is developed that leukaemia 'clustering' as well as national leukaemia incidence may be related to non-occupational exposure to benzene formed by petrol combustion and resulting from petrol evaporation. The possible association between exposure to fuel vapours, internal combustion products and cancer merits much closer examination than it receives at present.