Despite the advances in therapy, the high incidence, high mortality, and prematurity of coronary heart disease demonstrate the need for prevention. Measurement of a series of easily determined risk factors permits the early recognition of subjects at risk with remarkable reliability. However, reduction of risk factors affords protection against the illness only if they are causally connected with the disease mechanisms. The major evidence for linking atherosclerosis and its consequences with risk factors is reviewed. Particular attention is focused on serum lipids and the "lipid theory", smoking, elevated blood pressure, and physical inactivity, which are, on the basis of current knowledge, not only the most important factors but those most readily influenced by changes in daily living habits. Among the multiple risk factors mention is also made of obesity, diabetes, psychosocial stress, and hereditary predisposition. The probability of a causal relationship between risk factors and disease mechanisms justifies every effort to prevent the development of these precursors, or to treat them prophylactically if already present.