Two hundred consecutive stereotactic procedures were performed. These were 153 biopsies and 43 therapeutic procedures. The latter included aspiration of craniopharyngiomata, arachnoid cysts, deep-seated abscesses, and the destruction of colloid cysts. Cryogenesis was used to treat an arteriovenious malformation (AVM) in one case. In four cases, the system was used to locate lesions during open surgery. Leksell stereotactic systems were used throughout. The ages of the patients ranged between 12 months and 83 years. Overall, in 35% of biopsies the preoperative diagnosis was not confirmed. When the provisional diagnosis was glioma 70% were confirmed; 21% of them were benign, with 9% some other form of malignancy. Biopsy provided tissue on which a histological diagnosis could be made in 140 procedures. In eight cases the biopsy was repeated and in seven cases a positive diagnosis was subsequently made. Mortality was 1%; there was transient deterioration postoperatively in 5% and prolonged deterioration in 1%. These observations suggest that stereotactic surgery is far superior to freehand burr hole biopsy in the management of these lesions. No radiation treatment should be considered without histological confirmation.