Quantitative tumour growth into the brain (stage of the disease) and qualitative tumour evolution in the time (progression) are the two basic aspects of the natural history of the cerebral neoplastic disease. Recently the development of neuroradiological imaging (CT and MR) and the progress in biopathology of the nervous system tumours introduced new concepts like heterogeneity of neuroepithelial tumours or evolution to anaplasia. The findings obtained in 159 neuroepithelial tumours studied with stereotactic biopsy from 1980 to 1987 are presented. Most of them were glioblastomas (n = 43; 27%) and astrocytic tumours (n = 81; 50.9%). Twenty-nine cases of fibrillary astrocytomas (35.8% of all astrocytic tumours) showed focal anaplasia (progression). In 10 out of the 43 glioblastomas (23.3%) signs of astrocytic differentiation were clearly evident (secondary glioblastoma?). Our data confirm that neuroectodermal tumours, particularly of astrocytic series, undergo progression through anaplasia, which may be at the beginning a circumscribed phenomenon (focal anaplasia). The staging of the disease (tumour growth) into cerebral nervous system in some cases can not be correctly expressed through the neuroradiological imaging. Sometime CT scan may underestimate the true extension of the lesion. On the contrary, MR may overrate the extension of the lesion. Such mistakes in evaluation of tumour staging may be corrected through seriate stereotactic biopsy.