CEREBELLAR MEDULLOBLASTOMA is one of the most common types of primary intracranial tumors in children. The treatment of this tumor, unfortunately, is considered as “one of the darkest chapters in pediatric neurosurgery” (1). Cushing (2) reported an average survival of five and six-tenths months in 14 patients with medulloblastoma who received no postoperative radiotherapy. Ingraham and Matson (3) stated that “to our knowledge, no patient with cerebellar medulloblastoma has ever been cured.” Although medulloblastoma has long been known as the most sensitive primary brain tumor to ionizing radiation, the radiotherapy results for this tumor with respect to long-term survival is still very disappointing. In order to better evaluate the response of medulloblastoma to irradiation and to improve the survival rate, the operative records of 100 consecutive cases of tissue-proved medulloblastoma at the Neurological Institute, Columbia-Presbyterian Medical Center were arbitrarily taken in a period from 1940 to 1967 and were reviewed as to their primary location, extent of invasion, and gross subarachnoid seeding in correlation with radiation dose, technic, and survival results when possible. The age and sex distribution in this series is shown in TABLE I. Since clinical staging is inaccurate and sometimes misleading in many inaccessible human cancers, we have sought to develop an operative staging system for medulloblastomas with the International TNM-staging designation. The letter T stands for the primary tumor, subdivided to T1, T2, T3, T4, according to its size and the extent of involvement. Since there are no lymphatic channels in the brain, the letter N is omitted in our staging. The letter M stands for metastasis. In the cases of medulloblastoma, the cerebrospinal fluid pathway is the most important route of metastasis. The extent of metastasis is subdivided to Mo, Ml, M2, M3. Our proposed operative staging system for cerebellar medulloblastoma is as follows (Fig. 1): T1—Tumor less than 3 em in diameter and limited to the classic midline position in the vermis, the roof of the fourth ventricle, and less frequently to the cerebellar hemispheres. T2—Tumor more than 3 em in diameter, further invading one adjacent structure or partially filling the fourth ventricle. T3—It may be subdivided into T3a and T3b. T3a: Tumor further invading two adjacent structures or completely filling the fourth ventricle with extension into the aqueduct of Sylvius, foramen of Magendie, or foramen of Luschka, thus producing marked internal hydrocephalus. T3b: Tumor arising from the floor of the fourth ventricle or brain stem and filling the fourth ventricle.