The justifications for the dramatic expansion of the prison population in recent decades have focused on public safety. Prior research on the efficacy of incarceration offers support for such claims, suggesting that increased incarceration saves lives by reducing the prevalence of homicide. We challenge this view by arguing that the effects of mass incarceration include collateral infant mortality consequences that call into question the number of lives saved through increased imprisonment. Using an instrumental variable estimation on state-level data from 1978 to 2010, this article simultaneously considers the effects of imprisonment on homicide and infant mortality to examine two of the countervailing mortality consequences of mass incarceration. Results suggest that while incarceration saves lives by lowering homicide rates, these gains are largely offset by the increases in infant mortality. Adjusted figures that count the number of increased infant deaths attributable to incarceration suggest that the mortality benefits of imprisonment over the past three decades are 82% lower than previously thought.