In the context of health campaigns, interpersonal communication can serve at least 2 functions: (a) to stimulate change through social interaction and (b) in a secondary diffusion process, to further disseminate message content. In a 3-wave prospective study of 1,079 smokers, the authors demonstrate that mass media messages (antismoking campaigns and news coverage relevant to smoking cessation) have an indirect effect on smoking cessation intention and behavior via interpersonal communication. Exposure to campaigns and news coverage prompts discussion about the campaigns, and, in turn, about smoking cessation. Interpersonal communication regarding smoking cessation then influences intention to quit smoking and attempts to quit smoking. The study finds evidence not only for the social interaction function of interpersonal communication, but also for the secondary diffusion function. A substantial number of smokers who are not directly exposed to the antismoking campaigns are nevertheless indirectly exposed via communication with people who have seen these campaigns. These results imply that encouragement of interpersonal communication can be an important campaign objective.