Diagnosis, treatment, and prognostication in brain stem astrocytomas have been hindered by the occurrence in the same site of two distinct pathological entities-fibrillary and pilocytic astrocytomas. The small size of the specimens from this region adds an additional confounding factor in tumor classification. Nevertheless, histological assignment to either of these two prognostically different categories is often possible, especially if the importance of this distinction is recognized. In the face of a nonspecific histological diagnosis, e.g. "low-grade astrocytoma', certain radiographic and clinical features may, in combination with the pathological findings, be useful in tumor subclassification.