Between 1978 and 1984, 227 patients affected from glial tumors underwent serial stereotactic biopsy. Correct prognostic indications have been obtained in 85% of the entire series. In 10.5%, the stereotactic procedure provided only the diagnosis of the nature of the tumor, while the histological grading and prognosis remained unknown. The operative mortality was 2.6% and the permanent morbidity included 3.9% of patients who complained about worsening of the preoperative neurological conditions. The methodology of stereotactic biopsy is described in detail. The results are discussed with particular reference to the prognostic value of serial stereotactic biopsies.