This paper is concerned with the development of preventive medicine in the field of Coronary Heart Disease. It is based on an in-depth, ethnographic investigation into the popular culture of prophylactic behaviour carried out in South Wales (U.K.) during 1988 and 1989. The focus of the data and analysis presented here is the operation of cultural norms and practices related to the understanding and explanation of the cause and distribution of illness and death from heart ailments. The paper illustrates how the everyday cultural practice of 'lay epidemiology' is involved in accounting for illness misfortune and in assessing the potential benefits of prophylactic behaviour change. A central issue dealt with here is the relationship of lifestyle to environment in the popular understanding of chronic disease. Lay notions of luck, fate, destiny, randomness and chaos in the distribution of heart disease are explored. In conclusion, some implications for health education in this field are put forward.