e13530 Background: Standard therapy for patients with newly diagnosed glioblastoma (GBM) includes concurrent temozolomide (TMZ) and radiation (RT) followed by six monthly cycles of adjuvant TMZ. Although TMZ improves survival for MGMT methylated patients, it is unclear if this benefit is from the TMZ given with the radiation, the adjuvant TMZ, or if both are required. This retrospective study was designed to identify MGMT methylated patients who started treatment with near-normal functional status, completed concurrent RT+TMZ, but did not receive adjuvant TMZ to estimate if their survival was significantly less than expected. Methods: We reviewed charts from 190 adults diagnosed with GBM at Johns Hopkins Hospital from 2013 to 2015 recording their Karnofsky Performance Score (KPS), extent of resection, MGMT-methylation and IDH1 mutation status, and treatment records. We selected patients with MGMT-methylated GBM who received concurrent RT+TMZ without adjuvant TMZ for this study. Patients with a KPS < 70 or severe post-radiation complications were excluded. Results: Six patients met the selection criteria. They had a median age of 65 years, average KPS of 80, 2/6 had a gross total resection, and all were IDH1 wildtype. Their overall survival ranged from 17 to > 46 months. Conclusions: The overall survival of these patients who received no adjuvant TMZ is very similar to the median of 23 months reported in the literature for MGMT methylated patients who received adjuvant TMZ. This preliminary data suggests that a prospective study in MGMT methylated patients comparing concurrent RT+TMZ followed by either standard adjuvant TMZ or observation could be considered without placing patients in the experimental arm at excessive risk.[Table: see text]