Glial fibrillary acidic protein (GFAP) as a diagnostic and prognostic serum marker for glioblastoma multiforme (GBM) could improve both diagnosis and clinical management of brain tumour patients. To evaluate whether the serum concentration of   GFAP may indicate glioblastoma multiforme (GBM) in patients with single supratentorial space–occupying lesions, we prospectively examined 27 consecutive patients with histologically proven GBM, World Health Organization (WHO) grade IV, 6 patients with anaplastic astrocytoma (WHO grade III), 9 patients with anaplastic oligodendroglioma(III), 11  patients with diffuse astrocytoma (WHO grade II), 4 patients with a single cerebral metastasis and 7 healthy controls. Serum was taken from the patients before & after tumour resection.  Serum GFAP levels were determined using a commercially available ELISA test and 25 were detectable in out of the 27 GBM patients.   The levels were significantly elevated compared with those of the non–GBM tumour patients and healthy controls.  Non–GBM tumour patients and all healthy subjects showed zero serum GFAP levels. There was a significant correlation between serum GFAP level and GBM detection and also between the survival period and post operative GFAP levels. A serum GFAP level of  >0.11ng/ml was 78% sensitive and 98% specific for the diagnosis of GBM in patients with a single supratentorial mass lesion in this series. Post resection serum levels had a mean of 0.197ng/ml & levels >1.2 ng/ml were associated with survival period of less than one year. Therefore, it can be concluded that serum GFAP constitutes a diagnostic & prognostic biomarker for GBM.