A total of 474 adult patients with malignant glioma (astrocytoma) grade 3 or 4 were randomised into an MRC study (BR2) comparing 45 Gy (in 20 fractions over 4 weeks) with 60 Gy (in 30 fractions over 6 weeks) of radiotherapy given post-operatively. Using 2:1 randomisation, 318 patients were allocated the 60 Gy course and 156 the 45 Gy course. Adjuvant chemotherapy was not given. The results show that a 60 Gy course produces a modest lengthening of progression-free and overall survival. They suggest a statistically significant prolongation of median survival from 9 months in the 45 Gy group to 12 months in the 60 Gy group (hazard ratio = 0.75, chi 2 = 7.36, d.f. = 1, P = 0.007). Over 80% of patients reported no morbidity from the radiotherapy, and there was no evidence of increased short-term morbidity in the higher dose group. Late morbidity was not assessed. A prognostic index defined in a previous MRC study was validated in this new cohort. It identifies a group of patients (20% of the total) with a 2 year survival rate of 28% (95% confidence interval 19% to 38%). It was apparent that the survival advantage to the higher dose was maintained even in the poorest prognostic groups defined by this index.