Determination of plasma 24S-hydroxycholesterol, which is produced almost exclusively in the brain and is released only in small amounts into circulation under physiological conditions, might be a marker to monitor non-invasively the time course and the extent of the disintegration of the blood-brain barrier after cerebral ischemia. We investigated the plasma concentrations for 4 to 10 days of 24S-hydroxycholesterol, and compared the concentration with the liver-specific oxysterol 7alpha-hydroxycholesterol, the ubiquitously produced 27-hydroxycholesterol, and cholesterol itself in six patients who were admitted to the hospital within 24 h after symptoms of stroke. Quantification of oxysterols was performed by isotope dilution mass spectrometry and cholesterol by gas-liquid chromatography. Initial concentrations of cholesterol, 24S-, 7alpha-, and 27-hydroxycholesterol in stroke patients were not different from data of healthy controls given in the literature. During the following days, no changes could be observed in the concentrations of cholesterol nor in the other oxysterols. Particularly the brain specific 24S-hydroxycholesterol was very constant and showed only minimal changes. Furthermore, comparison of patients with extended or small lesions did not reveal any differences in the concentrations of oxysterols. Therefore, circulating levels of 24S-hydroxycholesterol are supposed to be only of limited value for monitoring the brain-blood barrier function in patients with acute ischemic stroke.