Using administrative data on workers' compensation claims in Michigan combined with data collected from a sample of workers identified by physicians as having work-related pain in their backs, wrists, hands, or shoulders, this article provides evidence that a substantial number of potentially eligible workers do not file workers' compensation claims. Multivariate analysis identifies the effects of various factors on the probability of filing a workers' compensation claim, conditional on having a work-related health problem. We find that the severity of the worker's condition and the worker's general health are the most important determinants of the decision to file, and that the generosity of wage loss benefits also affects the decision of an eligible worker to file. Finally, claims propensities vary considerably across workplaces, holding all other measured factors constant.