Some neurosurgeons state that intra-axial tumors may be resected with a low risk of neurological deficit if the tumor removal stays within the confines of the grossly abnormal tissue. This is thought to be so even when the lesion is presumably located in a functional area, providing that the adjacent normal-appearing cortex and subcortical white matter are not disturbed. This retrospective analysis presents evidence that this view is not always correct, because functioning motor, sensory, or language tissue can be located within a grossly obvious tumor or the surrounding infiltrated brain. Intraoperative stimulation mapping techniques identified 28 patients, ranging in age between 22 and 73 years, who showed evidence of functional tissue within the boundaries of infiltrative gliomas, as identified by correlation with computed tomography and magnetic resonance imaging scans, intraoperative ultrasound, gross visualization, and histological confirmation. Direct stimulation mapping of cortical and subcortical portions of the tumor during resections identified motor, sensory, naming, reading, or speech arrest function. Nineteen patients had new or worsened neurological deficits immediately after the operation, but after 3 months, only 6 continued to show new deficits whereas 18 showed no deficits and 2 improved. These results demonstrate that regardless of the degree of tumor infiltration, swelling, apparent necrosis, and gross distortion by the mass, functional cortex and subcortical white matter may be located within the tumor or the adjacent infiltrated brain. Therefore, to safely maximize glioma resection in these functional areas, intraoperative stimulation mapping may be used to identify functional cortical or subcortical tissue within, as well as adjacent to, the tumor, thus avoiding permanent injury.