Magnetic resonance imaging (MRI) is an important diagnostic tool for glioblastoma, with almost all cases showing characteristic imaging findings such as a heterogeneous-ring enhanced pattern associated with significant edema. However, MRI findings for early-stage glioblastoma are less clear. In this study, a retrospective review of MRI findings in five patients showed slight T2WI signal changes on initial scans that developed into typical imaging findings of a ring-like or heterogeneously enhanced bulky tumor within 6 months. The diagnoses based on initial MRI were low grade glioma in three cases, venous thrombosis in one case, and uncertain in one case. Four cases were treated with gross total resection, while one case underwent biopsy. Immunohistochemical examinations showed that two cases were p53-positive, and that all cases were IDH1 R132H-negative and had overexpression of EGFR. FISH analysis showed that all cases were 1p19q LOH-negative. De novo glioblastoma was the final diagnosis in all cases. Our results show that initial MRI findings in early-stage glioblastoma of small ill-defined T2WI hyperintense lesions with poor contrast develop to bulky mass lesions with typical findings for glioblastoma in as short a period as 2.5 months. The early MRI findings are difficult to distinguish from those for non-neoplastic conditions, including ischemic, degenerative or demyelinating processes. Thus, there is a need for proactive diagnosis of glioblastoma using short-interval MRI scans over several weeks, other imaging modalities, and biopsy or resection, particularly given the extremely poor prognosis of this disease.