We performed functional MRI with several semantic tasks to visualize parietal lobe functions. Twenty subjects were studied, including 9 normal controls and 11 patients with brain lesions. The calculation task was to simply add 3 numbers projected on a screen. Functional MRI showed the active pixels in the bilateral interparietal sulcus (especially the dominant side) and mean values and standard deviation of left/right ratios of active pixels in the interparietal sulcus were 1.63 +/- 0.57 in normal controls, and 1.64 +/- 0.72 in neurosurgical patients. In 3 patients, postoperative functional MRI well reflected sequential changes of their calculation ability. This technique is simple to apply to evaluate the severity of dyscalculia not only for parietal lobe lesions, but also for other diseases, such as developmental dyscalculia, autism and dementia.