Radiochemotherapy (RT) with concomitant followed by monthly temozolomide (TMZ) chemotherapy is the gold standard for the treatment of glioblastoma (GBM) patients. GBM patients can experience transient radiological deterioration after concurrent RT/TMZ that stabilizes or even resolves after additional cycles of adjuvant TMZ, a phenomenon defined as radiological pseudoprogression. The aim of this retrospective study was to identify a reliable marker associated with pseudoprogression processes. Patients with histologically proven newly diagnosed GBM were identified from a retrospective database between 2005 and 2009. Predictive factors for pseudoprogression were analyzed from clinical, radiological and biological data. Of the 130 analyzed patients, 63 underwent RT/TMZ treatment followed by cycles of TMZ and were evaluated for radiological responses every two months by magnetic resonance imaging. Early progression was confirmed in 52% (33/63) of the patients, and, within this group, 21% (7/33) displayed evidence of pseudo-progression. The predictive factors were evidenced in terms of clinical or radiological findings. In GBM patients, the level of cellular proliferation (Ki67 indices) emerged as a statistically significant prognostic marker for distinguishing pseudoprogression from actual progression. Our observation, suggesting that GBM associated with a high level of cellular proliferation may differentiate tumor progression from pseudoprogression, warrants further investigation in a large multi-center prospective study.