A large literature following Ruhm (2000) suggests that mortality falls during recessions and rises during booms. The panel-data approach used to generate these results assumes that either there is no substantial migration response to temporary changes in local economic conditions, or that any such response is accurately captured by intercensal population estimates. To assess the importance of these assumptions, we examine two natural experiments: the recession in cotton textile-producing districts of Britain during the U.S. Civil War, and the coal boom in Appalachian counties of the U.S. that followed the OPEC oil embargo in the 1970s. In both settings, we find evidence of a substantial migratory response. Moreover, we show that estimates of the relationship between business cycles and mortality are highly sensitive to assumptions related to migration. After adjusting for migration, we find that mortality increased during the cotton recession, but was largely unaffected by the coal boom. Overall, our results suggest that migration can meaningfully bias estimates of the impact of business-cycle fluctuations on mortality.