The Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive disorder characterized by accumulation of cholesta-5,7-dien-3 beta-0l caused by a deficiency of the enzyme desaturating this sterol to cholesterol. In addition to other unusual sterols recently found in plasma of patients with SLOS, namely cholesta-5,8-dien-3 beta-ol and 19-nor-cholesta-5,7,9 (10)-trien-3 beta-ol we have detected a trienol and we describe here its identification as cholesta-5,7,9 (11)-trien-3 beta-ol by GC-MS and by comparison with a synthetic standard. We tested the possibility that the trienol may be formed by radical oxidation of cholesta-5,7-dien-3 beta-ol accumulated in plasma of patients with SLOS because it is known to be formed by decomposition of 7-hydroperoxy-cholesta-5,8-dien-3 beta-ol, which is a product of cholesta-5,7-dien-3 beta-ol photooxidation. Incubation of cholesta-5,7-dien-3 beta-ol with rat liver microsomes in the presence of ADP/Fe2+ and NADPH gave rise to a number of oxygenated sterols. Among these, analysis by particle-beam LC-MS under CI conditions indicated the presence of 7-hydroperoxy-cholesta-5,8-dien-3 beta-ol and of cholesta-5,7,9(11)-trien-3 beta-ol which is known to derive from the oxidation of the 7-hydroperoxide. From these results we conclude that cholesta-5,7-dien-3 beta-ol accumulated in tissues of patients with SLOS may be oxidized by oxygen radicals giving rise to oxygenated sterols. Some of these compounds may be toxic and may contribute to worsen the pathological picture in patients with SLOS.