We have used cortical mapping during local anesthesia to help us define the limits of safe resection in neurosurgical patients undergoing tumor resection for lesions near language, somatosensory, or motor areas in the brain. Tumors located near the language areas in the frontal or temporal cortex were safely resected by first establishing the spatial limits for speech in these areas. In one patient with a tumor underneath the hand representation area of the right motor and sensory cortices, we also were able to define which cortical zones had to be avoided if a postoperative deficit secondary to tumor resection were to be averted. Brain swelling was not a problem; patients willingly cooperated during the cortical mapping procedure and were able to define accurately the kinds of sensory or motor changes that occurred when neurostimulation was used. It is possible safely to biopsy or resect tumors that previously might have been considered inoperable; intraoperative mapping is a useful surgical adjunct.