Recognition of the local nature of glioblastoma has generated an increasing interest in treatment using radioactive implants (interstitial brachytherapy). A key issue in such implantation is the configuration of the radiation field in relation to the resected tumor. In particular, should radiation be provided to the area from which the tumor has been resected? To clarify this issue, we evaluated patterns of tumor regrowth into this resected area in 62 patients. Three patterns of computed tomographic scan-documented tumor regrowth were recognized: preferential (regrowth to refill the resected area only), circumferential (regrowth into the resected area and previously uninvolved contiguous brain) and away (local regrowth into noncontiguous brain, sparing the surgical bed). Regrowth of the tumor 6.3 to 6.8 months after resection was seen in 59 of 62 patients (95.2%). Preferential regrowth was seen in 32 of 62 patients (51.6%), and circumferential regrowth was seen in 27 of 62 patients (43.5%). Regrowth away was seen in 3 of 62 patients (4.8%). Radiation fields planned for interstitial brachytherapy must adequately include the resected area because of the high incidence of tumor regrowth into that area.