Background: Glioblastomas (GBMs) are primary brain tumors that are very difficult to treat. Magnetic resonance imaging (MRI) is the reference tool for diagnosis, postoperative control, and follow-up of GBM. The MRI tumor contrast enhancement part serves as a target for surgery. However, there are controversial data about the influence of pre- and postoperative tumor volumetric MRI parameters on overall survival (OS). Methods: Data of 57 patients with GBM were analyzed retrospectively. All patients had maximum safe resection and standard adjuvant treatment. All patients underwent 1.5-T MRI with contrast in the first 24 hours postoperatively. The data of pre- and postoperative volumetric parameters were analyzed using the original software. Results: Correlation analysis between the postoperative volume of the tumor contrast enhancement part and the patient's OS revealed a significant level (on the Chaddock scale) of inverse correlation. Residual tumor volume associated with OS of >6 months was determined as <2.5 cm3. The mortality risk in the first 6 months after tumor resection is 3.4 times higher when the tumor remnant is >2.5 cm3 (risk ratio, 3.4; P = 0.0002). Conclusions: The volume of MRI contrast-enhancing GBM remnants after surgery, automatically measured by the software, was a significant predictor for early postoperative progression and death.