Low-grade astrocytomas in adults are uncommon malignant neoplasms of the central nervous system which are fatal in the great majority of patients despite a lack of aggressive histologic features. Several series of such patients have been previously reported, but prognostic factors have not been fully identified. Between 1960 and 1986, 50 patients with low-grade astrocytomas have been treated with megavoltage radiation at the Naval Hospital, Bethesda, MD, following surgical biopsy or excision. Overall actuarial survival at 10 years for the entire treated group was 32%, similar to other series. The most significant prognostic factor was patient age, with decreasing survival for each age decade and a highly significant difference in survival between patients less than age 40 compared to older patients (p = .0017). The era of treatment (before or after 1978) was also an important prognostic indicator (p = .057), largely due to an effect of dose, although better tumor localization in the CT era may also have played a role. There was a trend toward increasing survival with increasing dose of radiation, although not reaching statistical significance at the p = .05 level on multivariate analysis. Patient sex, extent of surgical resection, use of whole brain irradiation, and tumor grade did not significantly affect survival. In comparison, in a separate group of 10 patients who received surgery without radiation during the same period, all patients who were completely resected were long-term survivors, whereas none of those with incomplete resections survived longer than 6 years. © 1988.