Differences in food intake, smoking and drinking habits in the North and the South of Belgium have been studied with the aid of household data gathered by the National Institute of Statistics, Brussels, 1973-74. Consumption of sugar, vegetables, fruits, crude fibre and meat was almost identical between the regions. Consumption of bread, fish and salt intake were slightly higher in the North and alcohol consumption higher in the South. The major differences were located in fat consumption. Saturated fat as a percentage of dietary energy amounted to 15·8% in the North v. 18·5% in the South; polyunsaturated fat was, respectively for the North and South, 7·9% and 5·5%. Dietary cholesterol intake was 320 mg/day in the North against 400 mg/day in the South. The difference in serum cholesterol, calculated with the Keys formula, was 11·9 mg%, a value totally consistent with the observed values. The difference in saturated fat intake between the regions was almost entirely due to the difference of butter intake, thereby explaining why butter correlated so perfectly with mortality in different parts of Belgium. Similar correlations were found in France and Western Europe. The mortality trends in both regions were compared with the available data on fat consumption over the last 15 years. Again a decreasing intake of saturated fat (less butter and less common (hard) margarine) was associated with a decreasing coronary, cardiovascular and total mortality in both the North and the South. The time-related decrease discussed in the second part was quantitatively similar to one obtained in the first part from geographical differences, making a spurious association extremely unlikely. Similar dietary changes with identical results in terms of mortality have also been observed in the U.S.A. and Finland.