The cholesterol and cholestanol biosynthetic pathways and the control of cholesterolgenesis were investigated in skin fibroblasts, from patients with cerebrotendinous xanthomatosis (CTX) and from normal subjects, growth in a lipoprotein deficient (d less than 1.25 g/ml) medium. [3H]Acetate was added to the culture medium and its incorporation into sterols was assayed by both argentation and reversed-phase thin-layer chromatography (TLC). The labeling patterns were similar in both CTX and control cells with 3H being found, in order of increasing activity, in lanosterol, delta 7-cholestenol, and cholesterol. No 3H-labeled material at all, however, could be detected in the TLC mobility region corresponding to cholestanol. The ratio of cholestanol to cholesterol in the low density lipoprotein (LDL) subfraction from the plasma of individuals with CTX ranged from 1.4 to 5.3%, which is equal to or slightly greater than the ratio in whole plasma. Approximately 65-70% of the total plasma and LDL cholestanol and cholesterol were esterified. Since CTX-LDL, added to incubates of normal cells and normal LDL added to CTX fibroblasts suppressed HMG-CoA reductase activity and stimulated cholesterol esterification equally, and since 125-I-labeled control LDL was degraded with normal kinetics from the surface of CTX fibroblasts, both CTX-LDL and CTX fibroblasts LDL membrane receptors appear to be biologically normal. These results suggest that 1) cholesterol is synthesized in cultured CTX and control fibroblasts via delta 7-cholestenol, a C-24,25 saturated intermediate; 2) cholestanol is not synthesized in the skin of CTX patients but is transported there from the liver via the plasma LDL; and 3) CTX is not a disease associated with a defect of peripheral tissue LDL receptors.