The authors reviewed a personal series of 150 consecutive cases of intracranial meningiomas operated on between 1974 and 1988 with the aim of finding out the main prognostic factors determining surgical outcome. Severity of pre-operative clinical status and size of the tumour were found to be significant adverse factors, p < 0.001 and p < 0.01, respectively. In this article the authors stress on the role played in prognosis by pia mater vascularization of the tumour. When the tumour vascular supply predominated from pial-cortical arteries, in most cases cleavage could not be found in the arachnoid plane, but only in the subpial plane (because of incorporation of pia mater into the tumour "capsule"). Clinical consequences were that in the most eloquent areas (for example in the central region) a bad outcome--with transient or permanent deficit--frequently occurred when cleaving could not be performed in the arachnoid plane (p < 0.001). The neurological disorders were due to cortical and underlying sub-cortical ischaemia and haemorrhagic infarction. Participation of the pia mater in the tumour vascular supply can be predicted pre-operatively, directly on selective internal/external carotid angiograms, indirectly by the presence of an important peritumoural hypodensity on CT scan (which--according to our findings--is an indication of predominant pial-cortical vascular supply to the tumour). The positive correlation between cortical-pial supply (and its consequences) and tumour size (p < 0.001), pleads for surgery of meningioma at the earliest possible stage provided there are no contra-indications.