This article assesses the role of mass media markets in the diffusion of antismoking legislation in the state of Missouri in the US. Testing multiple diffusion theories, this study finds that media markets can facilitate the diffusion of policies among local jurisdictions by exacerbating intercounty competition. More attention should be given to media markets when assessing policy diffusion mechanisms at the local level. Media markets can be an important source for policy information conveyance among counties over a large geographic area. Furthermore, traditional diffusion studies that rely on geographical contiguity measures may not be completely capturing the existence of diffusion by excluding media markets.