To evaluate selected metabolic effects of plant fibers, we fed control and oat-bran diets in an alternating sequence to eight men with previously documented hypercholesterolemia. The two solid diets differed only in the inclusion of 100 g of oat bran in the test diet. We randomized diet sequences and the measured intakes of carbohydrate, protein, fat, and cholesterol were virtually identical on the two diets. Serum total cholesterol concentrations were stable on control diets whereas a progressive reduction was observed in seven men on oat-bran diets. On oat-bran diets, average reductions in serum total cholesterol concentrations were 13% (p less than 0.01, N = 8); plasma low-density lipoprotein cholesterol concentrations were 14% lower (p less than 0.05) while high-density lipoprotein cholesterol concentrations were not changed. Fasting and postprandial serum glucose, insulin, and triglyceride concentrations were similar on the two diets. Fecal excretion of total bile acids was 54% higher (p less than 0.001) on oat-bran diets than on control diets but neutral steroid excretion was slightly lower while on oat bran. Palatable and inexpensive high-fiber foods such as oat bran may have a role in the treatment of certain patients with hypercholesterolemia.