Workers' compensation systems attempt to evaluate claims for occupational disease on an individual basis using the best guidelines available to them. This may be difficult when there is more than one risk factor associated with the outcome, such as asbestos and cigarette smoking, and the occupational exposures is not clearly responsible for the disease. Apportionment is an approach that involves an assessment of the relative contribution of work-related exposures to the risk of the disease or to the final impairment that arises for the disease. This article discusses the concept of apportionment and applies it to asbestos-associated disease. Lung cancer is not subject to a simple tradeoff between asbestos exposure and smoking because of the powerful biological interaction between the two exposures. Among nonsmokers, lung cancer is sufficiently rare that an association with asbestos can be assumed if exposure has occurred. Available data suggest that asbestos exposure almost invariably contributes to risk among smokers to the extent that a relationship to work can be presumed. Thus, comparisons of magnitude of risk between smokers and nonsmokers are irrelevant for this purpose. Indicators of sufficient exposure to cause lung cancer are useful for purposes of establishing eligibility and screening claims. These may include a chest film classified by the ILO system as 1/0 or greater (although 0/1 does not rule out an association) or a history of exposure roughly equal to or greater than 40 fibres/cm3 x y. (In Germany, 25 fibres/cm3 x y is used.) The mere presence of pleural plaques is not sufficient. Mesothelioma is almost always associated with asbestos exposure and the association should be considered presumed until proven otherwise in the individual case. These are situations in which only risk of a disease is apportioned because the impairment would be the same given the disease whatever the cause. Asbestosis, if the diagnosis is correct, is by definition an occupational disease unless there is some source of massive environmental exposure; it is always presumed to be work-related unless proven otherwise. Chronic obstructive airways disease (COAD) accompanies asbestosis but may also occur in the context of minimal parenchymal fibrosis and may contribute to accelerated loss of pulmonary function. In some patients, particularly those with smoking-induced emphysema, this may contribute significantly to functional impairment. An exposure history of 10 fibre x years is suggested as the minimum associated with a demonstrable effect on impairment, given available data. Equity issues associated with apportionment include the different criteria that must be applied to different disorders for apportionment to work, the management of future risk (eg. risk of lung cancer for those who have asbestosis), and the narrow range in which apportionment is really useful in asbestos-associated disorders. Apportionment, attractive as it may be as an approach to the adjudication of asbestos-related disease, is difficult to apply in practice. Even so, these models may serve as a general guide to the assessment of asbestos-related disease outcomes for purposes of compensation.