Some recent studies have shown an extremely high level of serum cholesterol level among Finnish children. The aim of the present study was to test the effect of a "cholesterol lowering diet" on serum lipids and blood pressure among 36 children aged 8-18 years residing in two semirural communities in North Karelia, Finland. After a baseline period of two weeks on conventional diets, the proportion of energy derived from fat in the diet was reduced from the baseline level of 35% to 24%, and the ratio of polyunsaturated/saturated fats was increased from the baseline level of 0.18 to 0.61, during a 12-week intervention period. The children then changed back to their normal diet during a five-week switch-back period. The mean serum total cholesterol decreased 15% during the intervention, and then increased to nearly the initial level during the switch-back period. Similar changes were observed in HDL-cholesterol and also to some extent in apoproteins AI, AII and B. Blood pressure was not affected, although systolic blood pressure decreased during intervention. We conclude that the high level of serum cholesterol in Finnish children is to a great extent caused by the local dietary pattern and can be decreased by dietary modification.