The author identified profiles of chronic illness knowledge (i.e., heart disease, cancer, diabetes) in a community sample of American adults and examined the effect of sociodemographic influences on relations of illness knowledge to health practices and well-being. Participants were 181 women and 120 men who completed measures of illness knowledge, sociodemographics, personal health practices (e.g., diet, exercise, substance abuse, adaptive healthcare use), well-being (e.g., self-rated physical health, depression, social support), and perceived illness risk. Two-step cluster analyses performed on random subsets of the sample identified three levels of illness knowledge: low, medium, and high. Knowledge groups were differentiated on most measures of health practices, well-being and perceived illness risk. However, effects were substantially attenuated after controlling for differences in age and SES. Findings indicate that age and other sociodemographic factors are related not only to levels of illness knowledge but also to the application of knowledge in relation to health practices and well-being. (Contains 3 tables.)