The authors present specific clinical features of brain tumors occurring in immature brain in comparison with those of older infants and children. Twenty-six neonatal brain tumors, which accounted for 11.3% of 231 brain tumors diagnosed in the 1st year of life collected in a cooperative study in Japan, were analyzed. Although astrocytomas were invariably common tumors in each age group, comprising 25.0% for all pediatric brain tumors, teratomas were the most common type of neonatal brain tumors and accounted for 33.4% in this age group. Location of tumors in the immature brain was more striking, involving the posterior fossa region less commonly, as in 11.1% in neonatal, 29.5% in infantile, and 41.4% in all pediatric age groups. Surgical intervention was performed in 97.4% of the infantile and 79.1% of the neonatal cases, but total or subtotal resection of the mass was achieved only in 58.9% in the infants and 73.7% in the neonates. There was more strict limitation in performing the adjuvant therapies in the immature age groups. Radiation therapy and chemotherapy were given only in 32.0% and 27.6%, respectively, in the neonatal cases. The prognosis was extremely poor in the immature cases: 1-year, 5-year, and 10-year survival rates were 59.3%, 26.1%, 11.5% in the infants, and 43.5%, 23.8%, 7.1% in the neonates, respectively. Of the neonates, 76% and of the infants 73.1% were retarded. The present study delineates the characteristic clinical features of tumors involving immature brain and emphasized that establishment of more specific therapeutic modalities is urgent.