Research conducted by the U.S. Food and Drug Administration to select graphic warning labels for cigarette packs has been challenged as inadequate for demonstrating effects on smokers' beliefs about smoking. The present study tested the prediction that warnings alter risk perceptions and thoughts of quitting indirectly through a cognitive pathway (warning believability) and an affective pathway (worry about health), both of which are important for encouraging smokers to consider quitting. Using a national Internet panel, individuals who smoke were randomly assigned to view 1 of 3 types of warning labels: basic text only, graphic image with basic text, and graphic image with both basic and additional text elaborating on the reason for the health risk. Analyses were conducted to determine whether cognitive and affective reactions mediated effects on smoking-related outcomes. Images influenced perceived risk, immediate desire to smoke, and feelings toward quitting indirectly through affective reactions; elaborated text influenced these outcomes through cognitive believability, with little evidence of direct effects. Believability also enhanced positive feelings toward quitting among smokers who were less worried about health risks due to smoking. The findings indicate that (a) many effects of warnings on smokers' beliefs are mediated rather than direct, (b) both cognitive and affective responses are important mediators, and (c) elaborated text can help to increase effects of images through a cognitive pathway. Warning labels should be designed to maximize effects on these mediators in order to influence smoking outcomes.