Elderly and low Karnofsky performance status (KPS) patients have been excluded from most prospective trials. This retrospective study investigated glioblastoma treatment outcomes, including those of elderly and low KPS patients, and analyzed the prognostic factors using the medical records of 107 consecutive patients, 59 men and 48 women aged from 21 to 85 years (median 65 years), with newly diagnosed glioblastoma treated at our institute. There were 71 high-risk patients with age >70 years and/or KPS <70%. Based on the extent of resection, the patients were classified into 3 groups: more than subtotal resection (subtotal, n = 44), partial resection (partial, n = 29), and biopsy only (biopsy, n = 34). Median overall survival (OS) of all 107 patients was 13.5 months. Median OS was 13.2 months in the high-risk group. Median OSs were 15.8, 12.8, and 12.1 months in the subtotal, partial, and biopsy groups, respectively. Multivariate analysis of 73 patients in the subtotal and partial groups found age ≤65 years (p = 0.047), 60 Gy irradiation (p = 0.009), O⁶-methylguanine-deoxyribonucleic acid methyltransferase-negative (p = 0.027), and more than subtotal removal (p = 0.003) were significant prognostic factors. The median postoperative KPS score tended to be better than the preoperative score, even in the high-risk group. We recommend maximal safe resection for glioblastoma patients, even those with advanced age and/or with low KPS scores.