Incidence and mortality results are presented from an international controlled trial of multifactorial prevention of coronary heart disease, involving randomization of 66 factories (49 781 men) in U.K., Belgium, Italy and Poland (Cracow). Results for Poland (Warsaw) are not yet complete. Net average reductions in risk factors (all subjects) were 1.2% for plasma cholesterol, 8.9% for daily cigarettes, 0.4% for weight, 2.0% for systolic blood pressure, and 11.1% for a combined risk estimate. Greater reductions occurred in high-risk subjects (19.4% for the combined estimate). The net overall reduction in CHD rates was 7.4% (95% confidence interval -29 to +15%) for deaths (722 cases), and 3.9% (95% confidence interval -10 to +2%) for fatal CHD + non-fatal myocardial infarction (1502 cases). Among men aged 40-49 the reduction for this end-point was 15%; at ages 50-59 there was a small net increase. All-causes deaths after an early adverse trend showed 2.7% reduction overall. There were large differences between centres, ranging from a 5% net increase in CHD for U.K. to a decrease of 24% in Belgium. In Belgium the decrease both in CHD and in all deaths was significant at the 5% level. The effect on CHD in the different centres correlated broadly with their changes in risk factors. It is concluded that reduction in major coronary risk factors in industrial populations is possible, but it depends on adequate resources; the results support the hypotheses that CHD risk in middle-aged men is reversible and that community intervention can be beneficial.