Serum cholesterol-lowering effects of oat bran-enriched diets have been indicated in several studies in which oat bran was given several times a day. Concomitant changes in the daily diet, ie, a diminished energy intake or changes in the composition of fats in the diet, also have been reported and used to explain the hypocholesterolemic effect of oats. The present study was designed to replace only the conventional continental breakfast by a single oat bran cereal muesli containing 60 g of oat bran and to measure the effects of this dietetic modification on serum lipids in 13 patients with hypercholesterolemia type IIa. Compared with a 3-week baseline period, total serum cholesterol (7.38 +/- 0.35 mmol/L, mean +/- SEM) was reduced by 10.9, 8.4, and 9.7% in the first, second, and third week of oat bran ingestion (p < .01). High-density lipoprotein and low-density lipoprotein cholesterol as well as apolipoprotein A1 decreased to the same extent (8 to 11%; p < .05) during the oat bran period, whereas a pronounced reduction of 25.8% was seen for apolipoprotein B100 (p < .01), which is a major component of low-density lipoprotein. Dietary data obtained by 3-day food records at baseline, oat bran, and follow-up period did not show any differences between the study periods except for dietary fiber, which was increased from 21.9 g/day to 42.4 g/day (p < .002) during the test period because of the daily oat bran intake.(ABSTRACT TRUNCATED AT 250 WORDS)