Evidence from controlled clinical trials shows convincingly that reducing serum cholesterol levels by diet or drug treatment reduces the incidence of coronary heart disease. On a population basis, the most important effect of cholesterol lowering might be postponement of the first symptoms of disease rather than postponement of death, because most cardiac deaths occur at an advanced age. No enhanced cancer mortality is seen either in populations with low serum cholesterol levels or in patients who, through a genetic defect, have a low-density-lipoprotein cholesterol of zero. This makes it unlikely that cholesterol-lowering treatment as such promotes cancer. Still, specific side effects and toxicity of drugs need careful scrutiny, and diet remains the treatment of choice for mild hypercholesterolemia.