Food costs for 30 children under dietary treatment for familial hypercholesterolaemia were compared with those of 105 other Danish children. The daily intake of macronutrients and the daily cost of the diet for each child were calculated from dietary intakes and average prices of 365 different food items. The mean ± SE percentages of energy (E%) from fat in the diet of children with and without known familial hypercholesterolaemia were 23.6 ± 0.8 E% and 34.5 ± 0.5 E%, respectively (P < 0.001). The dietary costs per MJ in these two groups were 3.79 ± 0.12 Danish crowns (DKr) and 3.34 ± 0.05 DKr (P < 0.001), taking into account food wastage due to preparation and cooking. The cost per unit of energy increased with decreasing fat energy percentage of the diet for all children as one group (r = -0.37, P < 0.0001), as well as for the group of children without familial hypercholesterolaemia (r = -0.35, P < 0.001). Stepwise multiple regression analysis showed that the differences in cost per MJ between the groups could be explained primarily by differences in percentage of energy from fat. We conclude that a reduction of dietary fat from 35 E% to 25 E% may increase food costs by 10-20% for Danish children.