While a variety of effects of toxic chemicals are known in animals exposed both in the laboratory and in situ, it has proven more difficult to obtain definitive information relating harm to humans resulting from environmental contamination. Until quite recently it has been generally assumed that cancer was the human disease of greatest importance. In fact, the majority of regulations of exposure to toxic chemicals by most governments are designed on the basis of presumed cancer risk. The evidence that hazardous chemicals can cause cancer is strong, and concern of cancer risks is appropriate. However, recent evidence has triggered a reevaluation of the assumption that cancer is the sole disease of concern. New evidence has emerged suggesting that exposure to hazardous chemicals may lead to a variety of non-cancer endpoints, and that these effects may occur at low concentrations. Of particular concern is evidence for irreversible effects on the embryo and very young children which influence intelligence, attention span, sexual development and immune function. Some of these actions appear to be direct effects on the brain and other organ systems while others are mediated via disruption of endocrine systems. While these effects are subtle and difficult to quantify, the aggregated evidence is sufficiently compelling as to necessitate reevaluation of those health outcomes upon which regulations are based.