THE QUEST for the ideal inhibitor of cholesterol biosynthesis continues. To date, the clinical use of such inhibitors has disclosed definite disadvantages which include the accumulation in serum of triglycerides 1 or a precursor sterol, 24-dehydrocholesterol (desmosterol).2,3 The theoretically ideal inhibitor would be free of these disadvantages and would have its specific site of inhibition between β-hydroxy-β-methyl-glutaryl CoA and mevalonic acid. This has been suggested as the inhibition point in the negative feedback control of cholesterol synthesis by dietary cholesterol.4 The synthetic structural analog of cholesterol, 20,25-diazacholestenol dihydrochloride (SC-12937; 20,25-diazacholesterol) (Fig 1), was found to have its primary site of action at this enzymatic step and produced marked falls in serum cholesterol levels in experimental animals.5 Therefore, the compound was administered over a prolonged period of time to 13 patients with severe disorders of lipid metabolism and their serum lipids were studied. Materials and Methods The subjects