A simple classification system for central nervous system neoplasms occurring primarily in infancy and childhood and largely composed of undifferentiated neuroepithelial cells is proposed. Classification is based upon appearance of the tumor as determined by light microscopy, immunocytochemical techniques, and ultrastructural features without consideration for site of origin. This classification is based on the concept that neoplastic transformation of primitive neuroepithelial cells in subependymal zones at all levels of the central nervous system or pineal body may develop into tumors largely composed of similar cells. It therefore seems appropriate to call these neoplasms primitive neuroectodermal tumors and to use descriptive terms to indicate the direction of cellular differentiation, when it has occurred. Proposed terminology for the five subtypes of undifferentiated neuroepithelial round cell tumors is as follows: 1) Primitive neuroectodermal tumor, not otherwise specified (PNET, NOS), 2) PNET with glial differentiation, 3) PNET with ependymal differentiation, 4) PNET with neuronal differentiation, and 5) PNET with multi- or bipotential differentiation. If the tumor is located in the cerebellum, medulloblastoma may be added in parentheses; if in the pineal body, pineal parenchymal neoplasm may be added.