Three-strikes laws provide very long prison terms for certain criminals with prior convictions of serious violent crimes. It is likely that the laws increase homicides because a few criminals, fearing the enhanced penalties, murder victims and witnesses to limit resistance and identification. With a state-level multiple-time-series design, we find that the laws are associated with 10-12 percent more homicides in the short run and 23-29 percent in the long run. The impact occurs in almost all 24 states with three-strikes laws. Furthermore, there is little evidence that the laws have any compensating crime reduction impact through deterrence or incapacitation. Copyright 2001 by the University of Chicago.