We studied the relationship among the small-bowel length, the fecal excretion of bile acids and neutral steroids, and cholesterol synthesis in 16 patients with familial hypercholesterolemia before and 1 month after the ileal bypass operation. The mean small-bowel length measured during the operation was 6.3 +/- 0.3 (SEM) m before and 4.3 +/- 0.2 m after the bypass of the distal third of the small bowel. The preoperative fecal bile acid excretion was negatively correlated (r = 0.63, p less than 0.01) with the small-bowel length, whereas this correlation was insignificant for fecal neutral steroid excretion and cholesterol synthesis. Postoperative amounts of fecal bile acids and neutral steroids and cholesterol synthesis were negatively correlated with the length of the small bowel remaining in function (r = -0.52, -0.51, and -0.62, respectively). Our study suggests that fecal bile acid excretion and, hence, bile acid synthesis are related to the small-bowel length even under physiologic conditions.