The Smith-Lemli-Opitz syndrome is a genetic disorder characterized by typical clinical features including reduced myelination of both brain and peripheral nervous system and defective cholesterol biosynthesis. In patients this results in very low cholesterol concentrations and accumulation of cholesterol precursors in plasma, tissues, cultured cells and faeces. We now show that the cholesterol concentration in CSF of Smith-Lemli-Opitz patients is markedly decreased and that 7- and 8-dehydrocholesterol concentrations are highly increased in comparison to controls. Moreover, dietary treatment of patients with cholesterol seems not to affect CSF cholesterol concentration.