Previous research on word-of-mouth (WOM) diffusion has focused on the attributes of the sender, such as opinion leadership and innovativeness. This paper focuses on the social relationship between WOM senders and receivers and attempts to identify the optimal heterophily for WOM diffusion. The results of empirical analysis using both offline survey data and online blogosphere data provide evidence of optimal heterophily between the two; i.e., the WOM receiver wants to receive information from someone slightly more knowledgeable, not from a distant expert. The context of WOM communication is also examined in terms of the stage of hierarchical consumer behavior and sentiment. Finally, the patterns of influence are presented to depict the diffusion process. This analysis of optimal heterophily and the patterns of influence suggest that many grassroots influentials, rather than a few super influentials, play a central role in WOM diffusion.