In a previous study, we found that the extent of necrosis was the only radiological feature which correlated significantly with survival in patients with glioblastoma. The aim of this paper was to evaluate the variability and prognostic value of the extent of the necrotic area as seen on contrast-enhanced MRI and CT in a larger series. We studied 72 patients who underwent surgical removal of supratentorial glioblastomas and had CT and/or MRI with contrast medium before surgery; 38, all undergoing the same treatment (surgery plus radiotherapy), were followed clinically. Necrosis within the tumour varied greatly, ranging from none (only 1 case) to involvement of 76% of the tumour. Survival data in the subgroup suggested that only patients with a small area of necrosis (less than 35% of the tumour) had a significantly longer survival time. When necrosis involved more than 35% of the mass, patients had a shorter survival time, without any further correlation with the extent of necrosis.