Serum and cerebrospinal fluid (CSF) samples from 12 patients were analyzed for interleukin (IL)-6, soluble IL-2 receptor (IL-2R), and soluble CD8 levels in order to determine the immune activation profile following subarachnoid hemorrhage (SAH). Dramatically increased levels of IL-6 and moderate increases of soluble IL-2R were detected in the CSF in 11 of the 12 patients; slightly elevated levels of soluble CD8 were observed in six patients. The IL-6 levels were higher on Day 6 than on Days 3 and 9. The increases in IL-6, soluble IL-2R, and soluble CD8 levels in the CSF samples were not paralleled by increased values in the serum samples, and thus probably reflected an intrathecal synthesis of the cytokine. Passive transfer of IL-6 across the blood-brain barrier seemed not to occur since the serum and CSF levels of IL-6 showed a negative correlation. The findings suggest a severe inflammatory affection of the central nervous system that could be of importance in understanding the clinical course in patients following SAH.