YKL-40 is a newly discovered matrix protein thought to be secreted during the acute stages of inflammation. Clinical studies have revealed that YKL-40 has growth factor and potent migration factor activity for cells involved in inflammation and tissue remodeling processes. It has recently been speculated that YKL-40 may serve as a specific serologic marker of neutrophil function at the site of acute tissue inflammation. We aimed to quantify the levels of YKL-40 in both cerebrospinal fluid and serum of ten consecutive patients with aneurysmal subarachnoid hemorrhage and to speculate on the origin of this glycoprotein. The levels were also compared with ten control patients with hydrocephalus. We found that patients with aneurysmal subarachnoid hemorrhage had significantly higher YKL-40 levels in both cerebrospinal fluid and serum than controls. The authors believe that YKL-40 is expressed in cerebrospinal fluid due to stress on neural structures while a damaged blood-brain barrier allows entry of neutrophils and macrophages from the systemic circulation. (c) 2005 Elsevier Ltd. All rights reserved.