In order to determine whether cholestanol and bile acids are derived from the same precursor, key intermediates of both biosynthetic pathways beyond cholesterol were administered intravenously to a patient with cerebrotendinous xanthomatosis and to a control subject. After pulse-labeling with [4-¹⁴C]4-cholesten-3-one and [G-³H]7α-hydroxy-4-cholesten-3-one, cholestanol, cholesterol, and the two primary bile acids, cholic acid and chenodeoxycholic acid were isolated from specimens of bile. Each compound was purified by thin-layer chromatography and conclusively identified by gas-liquid chromatography-mass spectrometry. In other studies, the in vitro formation of 4-cholesten-3-one from cholesterol was measured in hepatic microsomal fractions prepared from a subject with cerebrotendinous xanthomatosis and from 3 control individuals. In all subjects, cholic acid and chenodeoxycholic acid were labeled with tritium, but neither cholesterol nor cholestanol contained this isotope. In contrast, ¹⁴C was detected in the cholestanol fraction with trace amounts in chenodeoxycholic acid, cholic acid, and cholesterol. Hepatic microsomes prepared from liver biopsy specimens obtained from a subject with cerebrotendinous xanthomatosis produced three times more 4-cholesten-3-one than the controls. The results indicate that 4-cholesten-3-one was converted primarily into cholestanol and 7α-hydroxy-4-cholesten-3-one into cholic acid and chenodeoxycholic acid. Neither ketonic steroid was transformed into cholesterol. The increased production of cholestanol in cerebrotendinous xanthomatosis may be accounted for by enhanced hepatic formation of 4-cholesten-3-one. 7α-Hydroxy-4-cholesten-3-one is a precursor of bile acids, but not of cholestanol.