A sample of 84 smokers attending health promotion clinics in a primary care setting completed questionnaires that assessed the main constructs of the theory of planned behavior, perceived susceptibility, and past cessation attempts. Regression analyses revealed that intention to quit smoking was primarily predicted by perceived behavioral control and perceived susceptibility. At 6-month follow-up, the making of a quit attempt was predicted by intention and the number of previous quit attempts, whereas the length of the quit attempt was predicted solely by the length of the longest recent quit attempt. The results suggest that interventions should focus on perceptions of susceptibility and control to increase smokers' motivation to quit. However, further work is required to identify the social cognitive variables that ensure that initial quit attempts are translated into longer term abstinence.