AbstractThis paper describes conditions under which epidemiologic data can provide estimates of the excess fraction (proportionate increase in caseload due to an exposure) and the etiologic fraction (fraction of cases caused by exposure). The excess fraction can be estimated under essentially the same conditions often cited for general study validity. In contrast, estimation of the etiologic fraction will usually require very specific nonidentifiable assumptions about exposure action and interactions, although one can derive simple lower and upper bounds for the fraction from survival comparisons. Since the etiologic fraction is equivalent to the probability of causation, our results have implications for injury compensation in lawsuits involving the probability of causation.