Bevacizumab and irinotecan have shown promising results in patients with recurrent glioblastoma multiforme (GBM), which traditionally carries a poor prognosis after first-line therapies have been exhausted. Retrospectively documenting the short-term effects of this chemotherapeutic regimen on recurrent GBM, as evidenced by comparative magnetic resonance images obtained two weeks prior to, and one-month following initiation of treatment, we hypothesize that peritumoral apparent diffusion coefficient (ADC) values will decrease on post-treatment scans. Brain MR data were collected from August 2005 to December 2006, in which post-contrast T1-weighted images demonstrated measurable enhancement or GBM tumor mass. Pre- and post-treatment MR images for ten consecutive patients were collected, each having failed temozolomide and radiation therapy. Pre- and post-treatment recurrent GBM bulk tumor and peritumoral T2 signal abnormality were measured in three dimensions. Diffusion of peritumoral T2 signal abnormality was evaluated on pre- and post-treatment ADC. All patients witnessed a significant decrease in tumor bulk ranging from 15.3% to 96.7% with a mean reduction of 48.2%, having received an average of two cycles of chemotherapy. FLAIR images demonstrated a mean volumetric reduction in peritumoral T2 signal abnormality of 44.3%. ADC measurements demonstrated an average reduction in peritumoral ADC of 20.6%, which was statistically significant (p-value < .005). Recurrent GBM tumor bulk demonstrated a 48.2% mean reduction, with corresponding decrease in peritumoral ADC values of 20.6%, suggesting that ADC may represent a valuable metric in the evaluation of the chemotherapeutic response of recurrent GBM, when treated with bevacizumab and irinotecan.