BACKGROUND: Preclinical work and retrospective studies suggest that temozolomide chemotherapy in glioblastoma may be more effective when administered in the morning rather than the evening. Here we examine the effect of timing in a large cohort of patients in 2 contemporaneous randomized clinical trials. METHODS: We assessed toxicity and survival data in patients with newly diagnosed glioblastoma enrolled in the CENTRIC EORTC 26071-22072 ( RESULTS: In CENTRIC and CORE, CONCLUSION: In this post hoc analysis, we found no difference in outcome based on the time of TMZ administration. Bone marrow toxicity might occur more frequently when temozolomide is administered in the morning. Given the limitation to data from deceased patients only, these analyses should be viewed as exploratory only.