Glioblastoma multiforme (GBM) are morphologically heterogeneous tumors, with varying amounts of necrosis, and edema. Previous studies have shown that treatments incorporating the VEGF antibody bevacizumab can reduce edema and tumor burden in GBM. Additionally it has been suggested that bevacizumab regimen treatment reduces the percent of tumoral necrosis. Therefore we sought to (1) determine the time course of change in necrosis, tumor, and edema volume in patients who respond to bevacizumab regimen treatment and (2) determine if GBM that progress following a response to bevacizumab regimen treatment are morphologically different from their appearance at prior tumor progression. Therefore, we retrospectively assessed tumor, necrosis, and edema volumes on MRI scans from 15 patients with recurrent GBM who responded to bevacizumab regimen treatment, and had extended (>7 month) follow-up. We found that the median time to best tumor response was 158 days (range, 16-261, SD = 63). The median best response was 72.1% reduction in tumor volume and 72.8% reduction in peritumoral edema. Most tumors (77.8%) showed resolution of necrotic areas. The relative reduction of edema and necrosis was sustained, even in patients (n = 7) who developed tumor progression. Thus the mean ratio of edema-to-tumor volume at progression on bevacizumab regimen treatment was 38.4% lower than that for the same tumors seen on progression scans following prior chemotherapy. The percentage of necrotic tumor also was diminished following progression on bevacizumab regimen treatment. These findings illustrate the time course of changes in edema and tumor volume with prolonged bevacizumab regimen treatment, and support the conclusion that the morphology of recurrent GBM following bevacizumab regimen therapy is distinct from that on other chemotherapy.