Total mortality from cardiovascular disease in Denmark has decreased over the last 20 years for women and the last ten years for men. The possible role of simultaneous changes in serum total cholesterol has been investigated. Secular trends in serum cholesterol, high density lipoprotein cholesterol, and triglycerides, 1964-1987, are presented on the basis of four studies of 30-, 40-, 50- and 60-year-old men and women, some 8737 subjects in all. A significant decrease of 1% per year in total serum cholesterol (p less than 0.05) in both sexes and in all age groups up to 1982 was detected followed by a subsequent stabilization, 1982-1987. The decrease is not a result of methodological bias. The impact of storage at -20 degrees C for 13-24 months compared to immediate analysis of sera was studied as well as differences in analysis methods over time. The fall in population cholesterol levels might be associated with changes in polyunsaturated/saturated fat (P/S) ratio rather than total fat content of the diet, but other lifestyle changes have taken place as well.