Use of oat-supplement has been advocated to reduce serum cholesterol concentration. In order to study the effect of a daily dietary supplement of oats on lipid levels we performed three controlled clinical trials in healthy Norwegians and patients with hypercholesterolemia. The studies lasted 3-5 weeks, and oats were added to the diet in bread, breakfast cereals, porridge or crispbread. The serum cholesterol concentrations were reduced by 2.5-5% in four of the five groups with an oat-supplemented diet. No effect was found in the group who ate crispbread. Lipid levels remained unchanged in the three control groups. We conclude that oat products have a small but significant cholesterol-lowering effect when given as a supplement to the ordinary Norwegian diet.