One of the major tasks of epidemiology is to identify risk factors for disease. Our failure in this mission can be illustrated by examining the case of coronary heart disease. Since the end of the Second World War, coronary heart disease has been studied perhaps more aggressively than any other disease. During this period of tremendous international effort, a large number of seemingly important risk factors have been identified. The three that everyone agrees on are cigarette smoking, high blood pressure and high serum cholesterol. Dozens of other risk factors also have been proposed but not everyone agrees about them. Included here are such risk factors as obesity, physical inactivity, diabetes, blood lipid and clotting factors, stress, and various hormone factors. When all of these risk factors are considered together, they account for about 40 per cent of the coronary heart disease that occurs (Marmot and Winkelstein 1975).