Aim: To assess the value of perfusion magnetic resonance imaging (pMRI) in the differentiation of early pseudoprogression from true progression in glioblastoma multiforme (GBM) patients taking Temozolomide (TMZ) with radiotherapy (RT) treatment. Materials and Methods: Pre-RT and post-RT cranial pMRI scans of 23 GBM patients treated with RT-TMZ were reviewed. Relative cerebral blood volume (rCBV) and relative cerebral blood flow (rCBF) of the residual enhancing lesions were measured on serial pMRI scans and proportioned. ROC analysis was performed to determine a threshold ratio of decrease in rCBV and rCBF. Results: There were nine patients (39%) with signs of radiological progression, of whom six (67%) had real progression and three (33%) had pseudoprogression based on follow-up MRI studies, clinical parameters and/or pathology. Ratio of decrease was 2.928 in rCBV and 2.510 in rCBF in the pseudoprogression group, which were significant according to Mann-Whitney U test (p=0.02). Cut-off ratio of decrease value of 1.73 for rCBV and 1.62 for rCBF between pre-RT and post-RT pMRI study, could differentiate the presence of early pseudoprogression with 100% sensitivity and 100% specificity. Conclusion: Ratio of decrease in rCBV and rCBF is a reliable predictor of early pseudoprogression in GBM patients under RT-TMZ treatment.