Elevated levels of cholesterol synthesis are reported for several young children with homozygous familial hypercholesterolemia (HFH) and are considered to contribute directly to their hypercholesterolemia. In contrast, increased cholesterol production has not previously been found in adult patients with HFH. Using the fecal steroid balance technique, we studied rates of cholesterol and bile acid synthesis in a 24-yr-old man who had severe hypercholesterolemia typical of HFH and who lacked skin fibroblast low density lipoprotein (LDL) receptor activity. On an average diet (45% carbohydrate, 40% fat, 15% protein) mean +/- SEM cholesterol (24.8 +/- 1.4 mg/kg per d) and bile acid (11.1 +/- 1.6 mg/kg per d) excretion were approximately threefold higher than normal. When an isocaloric high carbohydrate, low fat diet (90.5% glucose oligosaccharides, 1.3% safflower oil, 8.2% crystalline amino acids was substituted, mean cholesterol (13.0 +/- 0.5 mg/kg per d) and bile acid (8.6 +/- 0.4 mg/kg per d) fell markedly. The decline in fecal steroid excretion was accompanied by modest reductions in plasma total and LDL cholesterol concentrations and by a softening of cutaneous xanthomata. Although the patient phenotypically and biochemically resembled the HFH state, his family pedigree was not noteable for hypercholesterolemia. While the patient's father had premature cardiovascular disease, his mother had no evidence of heart disease, had normal plasma total and LDL cholesterol levels, and had normal fibroblast LDL receptor activity. Likewise, the plasma cholesterol levels of three other members of the patient's family were normal. Despite the unusual genotypic background of this individual, however, the fecal balance data shows that elevated cholesterol and bile acid synthesis may occur in adult, as well as juvenile, patients with HFH and may be responsive to dietary control.