The perception of cancer risk motivates cancer risk reduction behaviors. However, common measurement strategies for cancer risk perceptions, which involve numerical likelihood estimates, do not adequately capture individuals' thoughts and feelings about cancer risk. To guide the development of novel measurement strategies, the authors used semistructured interviews to examine the thought processes used by smokers (N = 15) as they considered their cancer risk. They used grounded theory to guide systematic data coding and develop a heuristic model describing smokers' risk perception process that includes a cognitive, primarily rational process whereby salient personal risk factors for cancer are considered and combined, and an affective/attitudinal process, which shifts risk perceptions either up or down. The model provides a tentative explanation concerning how people hold cancer risk perceptions that diverge from rational assessment of their risks and will be useful in guiding the development of non-numerical measurements strategies for cancer risk perceptions.