The success of the Community Intervention Trial for Smoking Cessation (COMMIT) in changing smoking attitudes is examined by testing two primary hypotheses: (1) the priority of smoking as a public health problem increased more in the intervention communities than in the comparison communities, and (2) norms and values that support non-smoking increased more in the intervention than in the comparison communities. One community within each of 11 matched pairs was randomly assigned to receive a 4-year (1989-92) community-based smoking control intervention. Community attitudes towards smoking were measured primarily by cross-sectional surveys in 1989 (n = 9875) and 1993 (n = 14117) but a cohort (n = 5450) also provided attitude information. The main trial effect was on heavy smokers in the intervention communities who showed significantly more change in their beliefs about smoking as a public health problem. Despite the absence of an intervention-comparison difference, the magnitude of change in community-wide norms and values was related to the level of smoking control activities. In the cohort, light-to-moderate smokers in the intervention communities came to have stronger beliefs about smoking as a serious public health problem. COMMIT's impact on the beliefs of heavy smokers about the seriousness of smoking as a public health problem has important public health implications.