The current best standard care for glioblastoma still has limitations and unsatisfactory outcomes in patients with an unmethylated O-6-methylguanine-DNA methyltransferase (MGMT) promoter. Whether the effects of temozolomide are primarily due to its concomitant use with radiotherapy or are also mediated by their independent use in the adjuvant phase remain unclear. To validate the concomitant use of temozolomide in the standard protocol, we compared the overall survival of two prospective patient groups: one treated with radiotherapy alone followed by adjuvant temozolomide (RT → TMZ group) and the other treated with concomitant radiotherapy and temozolomide followed by adjuvant temozolomide (CCRT-TMZ group). Each patient in the RT → TMZ group (n = 25) was matched with two patients in the CCRT-TMZ group (n = 50) with respect to age, extent of resection, MGMT promoter methylation status, and postsurgical performance status to minimize the influence of confounding factors. In patients with MGMT promoter methylation, the CCRT-TMZ group showed superior overall survival (OS; median, 41.0 months) and progression-free survival (PFS; median, 24.0 months) compared with the RT → TMZ group. However, the OS and PFS did not differ between the CCRT-TMZ and the RT → TMZ groups in the patients without MGMT promoter methylation. Although this data is from a retrospective analysis using small number of patients, the study might indicate that concomitant use of temozolomide with radiotherapy is a crucial step in the standard treatment for glioblastoma patients with MGMT promoter methylation. And the use of temozolomide, either concurrently or by adjuvant after radiotherapy, remains a questionable value for those with an unmethylated MGMT promoter.