Background: Numerical processing is important in our everyday lives. However, very few attempts have been made to map the numerical processing function areas during lesion surgery. Objective: To identify and protect the cortical areas involved in numerical processing, the authors used the intraoperative brain mapping approach to study numerical processing areas in patients with parietal lobe tumors. Methods: During resection in patients with parietal lobe tumors, local anesthesia was administered and numerical processing mapping was performed. Our mapping procedures were conducted before glioma removal and included somatosensory, language and numerical processing tasks. We focused on the numerical processing task. Results: Different brain sites within the parietal lobe were detected to be specifically related to multiplication or subtraction processing. They displayed precise spatial distribution and overlapped with each other. No brain sites were found to be specifically related to numerical processing in the right hemisphere. Conclusions: To improve the quality of resection while minimizing the neurological deficits, functional boundaries of numerical processing areas should be considered during the removal of a parietal low-grade glioma. Moreover, only the left intraparietal sulcus is necessary for numerical processing, whereas the right intraparietal sulcus does not appear to be critically involved in numerical processing. Copyright (c) 2010 S. Karger AG, Basel