Epidemiologic perspectives on life-style modification and health promotion in cancer research. The clinical, patient-oriented focus of medicine and psychology is contrasted with the epidemiologic (public health) approach in assessing the role of life-style factors and health promotion in cancer research. The unifying host-agent-environment epidemiologic paradigm is applied to contemporary cancer prevention issues, principally smoking cessation and dietary modification, to demonstrate differences in inferences, prevention strategies, and research opportunities. An integration of population-based approaches with the dynamics of patient behavior and risks for cancer is especially salient when considering the role of psychosocial stress and personal and social resources. The social epidemiologic perspective, the study of the psychosocial determinants of physical health status, offers one approach for resolving the outlined differences in perspectives and is particularly relevant for understanding the etiology of life-style behaviors and how they might be altered. 