Petromyzonol, a rare bile substance, has been prepared from an abundant starting material, cholic acid. The critical conversion, the inversion of the configuration at C-5, was accomplished by oxidizing a suitably protected derivative of cholic acid to a 1,4-dien-3-one, which was then stereoselectively reduced to methyl allocholate. In a single oxidation step, the 3-hydroxy steroid was converted to the dienone by Barton's procedure using a catalytic amount of benzeneseleninic anhydride andm-iodoxybenzoic acid as the stoichiometric oxidant. The stereoselective reduction employed two steps: hydrogenation of the dienone to the saturated ketone (5α) in the presence of the Wilkinson catalyst, followed by reduction of the ketone with K-Selectride. Finally, methyl allocholate was reduced to petromyzonol with lithium aluminum hydride.