Cholesterol 5 alpha-hydroperoxide, the major product of O-1(2)-oxidation of cholesterol, readily undergoes acid-catalyzed (Hock) cleavage of the C5-C6 bond to yield the 5,6-secosterol ketoaldehyde and the product of its intramolecular aldolization. These cholesterol oxidation products have long been thought to arise solely from ozonolysis of cholesterol, prompting the recent suggestion that ozone is produced in humans following their identification in arterial and brain tissue extracts.