The incidence of coronary heart disease (CHD) can be reduced by preventive measures. We know what it costs to treat CHD, but lack even the most rudimentary estimates of what it might cost to reduce the incidence by means of population wide strategies of prevention. American and Australian experience shows that such strategies achieve results. Even in Britain, where official spending on prevention is relatively small, health publicity has succeeded in bringing about marked reductions in the household consumption of animal fats and in cigarette smoking. Yet we know virtually nothing about the cost of bringing about a given reduction in CHD mortality by preventive measures. Estimates of such costs would be useful in persuading public authorities to spend more on prevention.