Seventy-nine patients with tumors of the medulla oblongata, pons, and midbrain treated by radiation therapy between 1962 and 1987 were analyzed. According to histology obtained in 28 cases, 61% were high-grade astrocytomas and 39% were low-grade astrocytomas. The 5- and 10-year survival rates of 71 patients who completed radiotherapy were 17 and 15%, respectively. Various potential prognostic factors were analyzed, and patients with a midbrain tumor, with a low-grade tumor, and without cranial nerve paresis were found to have a better prognosis. Computed tomography (CT) was performed both before and after radiotherapy on 42 patients. The volume of the tumor estimated from the CT image was various, but the tumors could be classified into three types according to the pattern of contrast enhancement (CE). Twenty-three tumors showed no CE, 15 showed a diffuse CE, and 4 showed a ring CE. No-CE tumors were generally hypodense on plain scan and were mostly large, whereas diffuse- or ring-CE tumors were relatively small. A complete response (CR) or partial response was obtained with 77% of the no-CE tumors, 64% of the diffuse-CE tumors, and 50% of the ring-CE tumors. The CR rate was higher in the tumors with a smaller volume. The 5-year survival rates for diffuse-CE, no-CE, and ring-CE tumors were 33, 15, and 0%, respectively. The 5-year survival rate was 56% for tumors smaller than 6 cm3, 16% for tumors between 6 and 20 cm3, and 0% for tumors larger than 20 cm3. In conclusion, (a) ring-CE tumors exhibited a poor prognosis; and (b) a diffuse-CE tumor, or a small or middle-sized no-CE tumor is most likely to be controlled by conventional radiation therapy.