In 1993 the Smith-Lemli-Opitz (SLO) syndrome, known as a malformation syndrome characterized by certain stigma, turned out to be a metabolic disease with a defect in the last step of cholesterol biosynthesis. This led to the possibility of identifying affected individuals by biochemical methods and of increasing understanding of pathogenic mechanisms. Hopes of influencing the effects of the metabolic defect by dietary supplementation were raised and reports with some benefits of treatment have been published. This is a report of a 12-y-old girl with the SLO syndrome in an apparently progressive form. In addition to typical signs and well-known symptoms she has a verified polyneuropathy and precocious puberty. She has been treated with cholesterol and bile acids for 3 y, during which time the progressive course has been arrested. A notable effect has been the improvement of her polyneuropathy, verified by measurement of nerve conduction velocities. Possible mechanisms involved in the pathogenesis of her precocious puberty are discussed.