Objectives-To assess the concentrations of S-100 protein, myelin basic protein (MBP), and lactate, and the (CSF)/serum albumin ratio (Qalb) during intracranial neurosurgical procedures. Methods-Samples of CSF from 91 patients with various CNS diseases were obtained by aspiration of cisternal CSF at the beginning of surgery (before starting surgical manipulation of the brain) and concentrations of S-100 protein, MBP, and lactate, and Qalb were determined. At the same time blood was sampled for determination of serum S-100 protein concentration. Patients were divided into three groups according to the aetiology of their CNS disease (intracranial haemorrhage, n=11; benign intracranial mass lesion, n=52; malignant neoplastic disease, n=28). Radiological and intraoperative characteristics were documented. Results-In each of these three groups median values of all four CSF variables measured were raised. The occurrence of brain oedema and a midline shift correlated significantly with raised concentrations of MBP and Qalb. Breaching of the arachnoid layer, documented at surgery for benign lesions, correlated with higher concentrations of MBP, lactate, CSF S-100 protein, and Qalb. Conclusions-Intraoperative values of S-100 protein, MBP, lactate, and Qalb are increased in patients with intracranial haemorrhage, benign intracranial mass lesion, and malignant neoplastic disease. Breaching of the arachnoid layer and oedema is associated with higher concentrations of some of the aforementioned proteins. These biochemical data can serve as a basis for further research into CSF specific proteins. (J Neurol Neurosurg Psychiatry 2001;71:671-674)