Fractionated external beam radiotherapy is an important component of standard treatment for high grade glioma. Due to resource constraints, patients may experience delays in receiving treatment. The purpose of this study was to evaluate the effect of radiotherapy waiting time on survival in patients with high grade glioma. A retrospective analysis was performed of 172 patients with a histological diagnosis of WHO Grade 3 or 4 Astrocytoma who had undergone surgery at Wellington Hospital between 1993 and 2003, and who subsequently underwent radiotherapy. Time to radiotherapy after surgery varied from 7 days to over 16 weeks. Multiple Cox regression analysis showed that age, performance status, tumour grade, extent of surgical resection, radiotherapy dose, and time to radiotherapy from day of surgery were all independently related to survival. Every additional week of delay until the start of radiotherapy increases the risk of death (hazard ratio) by 8.9% (95%CI 2.0%–16.1%). A 6 week delay in starting radiotherapy (from 2 weeks post-op to 8 weeks) reduces median survival by 11 weeks for a typical patient. Delay in radiotherapy results in a clinically significant reduction in survival. These findings have implications for resource allocation and for the design of clinical trials.