Stereotactic biopsies from deep-seated brain tumors were carried out in 302 patients. There were more men (58 percent) than women. Peak of prevalence was in the second to fourth decade; the youngest patient was 18 months old. The tumors were deep-seated hemispheric in 36 percent, in the basal ganglia in 30 percent, in the diencephalic region, i.e., hypothalamus and suprachiasmatic region, in 21 percent, and in the midbrain and pineal region in 13 percent. The majority of lesions were gliomas (71 percent), the remaining being epithelial and various nonglial tumors. Operative mortality was 2.3 percent; transient deterioration was observed in 3 percent. Diagnosis was made instantly by cytological examination (smear preparations) and confirmed by conventional histological examination. In only 5 percent of the gliomas was a discrepancy found between the diagnosis from the smear preparation and the subsequent histological diagnosis. Integrating computerized tomography (CT) imaging techniques into the stereotactic technique makes it possible to take biopsies at exact and reproducible sites and with low risk. The findings suggest that exploratory craniotomies, risky free-hand punctures, and aspirations deep in the brain can no longer be advocated. No radiation treatment should be considered without histological confirmation.