A controlled intervention trial, with the purpose of testing the hypothesis that the incidence of coronary heart disease (CHD) could be decreased by the use of a serum-cholesterol-lowering (SCL) diet, was carried out in two mental hospitals near Helsinki in 1959-71. The subjects were hospitalized middle-aged women. One of the hospitals received the SCL diet, ie a diet low in saturated fats and cholesterol and relatively high in poly-unsaturated fats, while the other served as the control with a normal hospital diet. Six years later the diets were reversed, and the trial was continued another six years. The use of the SCL diet was associated with markedly lowered serum cholesterol values. The incidence of CHD, as measured by the appearance of certain electrocardiographic patterns and by the occurrence of coronary deaths, was in both hospitals during the SCL-diet periods lower than during the normal-diet periods. The differences, however, failed to reach statistical significance. An examination of a number of potential confounding variables indicated that the changes in them were small and failed to account for the reduction in the incidence of CHD. Although the results of this trial do not permit firm conclusions, they support the idea that also among female populations the SCL diet exerts a preventive effect on CHD.