The standard-of-care treatment for newly diagnosed glioblastoma changed in 2005, when radiation therapy plus temozolomide chemotherapy replaced radiation therapy alone. It is not yet clear how this change in treatment has influenced patient survival in routine clinical practice, or if a survival benefit extends to patients older than those enrolled in the trial. Data from the Surveillance, Epidemiology, and End Results (SEER) Program was analyzed to compare survival of adult glioblastoma patients diagnosed from 2000–2003 to patients diagnosed from 2005–2008, in order to evaluate pre-temozolomide and post-temozolomide periods. The Kaplan–Meier method and Cox proportional hazards models were used. 6,673 patients with glioblastoma diagnosed from 2000–2003 and 7,259 patients diagnosed from 2005–2008 were identified. Median survival times of all patients diagnosed in the 2000–2003 and 2005–2008 periods were 8.1 and 9.7 months, respectively. Amongst patients treated with surgery and a radiation-containing regimen, median survival was 12.0 months in 2000–2003 and 14.2 months in 2005–2008. In the temozolomide era, median survival times ranged from a high of 31.9 months in patients age 20–29 to a low of 5.6 months in patients age 80 and older. The survival of patients with newly diagnosed glioblastoma improved from 2000–2003 to 2005–2008, likely due to temozolomide use. However, median survival time after glioblastoma diagnosis in the SEER population remains well under one year, largely driven by poor prognosis in elderly patients.