There is now considerable evidence that peroxisomes not only have a role in cholesterol oxidation but also in cholesterol biosynthesis. Specifically, peroxisomes contain at least two enzymes necessary for the initial steps in cholesterol synthesis, i.e., thiolase and mevalonate kinase. The rate-limiting enzyme in cholesterol synthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase, is also localized in peroxisomes and exhibits a cyclic variation distinct from that of the reductase found in the endoplasmic reticulum. The largest concentration of cellular sterol carrier protein-2 is localized in peroxisomes as well as a number of enzymes required for the conversion of lanosterol to cholesterol. Furthermore, peroxisomes are involved in the in vitro synthesis of cholesterol and dolichol from mevalonate and have been shown to contain significant levels of apolipoprotein E, a major constituent of several classes of plasma lipoproteins. Moreover, cholesterol synthetic capacity is impaired in cultured skin fibroblasts obtained from patients with peroxisomal deficiency diseases.