A double-blind cross-over study was conducted during two 3-week periods to compare the effects of rapeseed oil and sunflower oil, enriching a normal diet, on the lipoprotein and fatty acid composition in healthy subjects. It was carried out in randomized order at residential schools, comprising 101 persons (mean age 29.2 years). The dietary fats used for cooking and as table margarine were prepared from rapeseed oil during one period and from sunflower oil during the other. No changes were made in the total fat content or other dietary nutrients. During both treatment periods the serum cholesterol (-4%, P < 0.001), LDL cholesterol (-5% to -7%, P < 0.01 and 0.001) and apolipoprotein B (-5%, P < 0.001) concentrations decreased significantly and to the same extent, while serum triglycerides, HDL cholesterol, apolipoprotein A-1 and lipoprotein (a) remained virtually unchanged. The content of 18:2 n-6 serum phospholipids was increased after the sunflower oil-enriched diet, and the contents of oleic acid (18:1 n-9), alpha-linolenic acid (18:3 n-3), and eicosapentaenoic acid (20:5 n-3) were increased after the rapeseed oil-enriched diet. The concentration of alpha-tocopherol increased and gamma-tocopherol decreased after the sunflower oil-enriched diet, less so after the rapeseed oil-enriched diet. It is concluded that substitution of mono- and polyunsaturated fats for saturated fats without any other dietary changes causes a significant improvement of the lipoprotein profile in healthy subjects. The rapeseed oil and sunflower oil fats were equally effective in this respect. The results also indicate that humans have a certain capacity to elongate and desaturate alpha-linolenic acid to 20:5 n-3 in vivo. Dietary fats based on rapeseed oil seem to be attractive alternatives to the more commonly used oils and fats rich in linoleic acid. Financial support from the Swedish Council for Forestry and Agricultural Research and the Swedish Margarine Industrial Association for Nutritional Physiological Research is gratefully acknowledged.