We recently completed a review of scientific evidence, particularly epidemiologic evidence, regarding the contribution of environmental and occupational exposures to the overall cancer burden in the US. We evaluated the efforts to estimate the proportion of cancer due to these involuntary exposures, including the ambitious effort by Doll and Peto and an update by a group of authors at the Harvard Center for Cancer Prevention. In this paper, we critique these efforts, and their resulting estimates of the proportion of cancer due to various factors. We also provide an alternative interpretation of the evidence and a caution against the very idea of attributing specific fractions or proportions of cancer to particular factors. We conclude by recommending that environmental and occupational links to cancer be given serious consideration by individuals and institutions concerned with cancer prevention, particularly those involved in research and public education. We support the new initiative in the European Union to evaluate chemicals more fully before they reach the market.