To investigate whether prefrontal function during a cognitive task reflects the severity of panic disorder, the prefrontal function during a word fluency task in 109 panic disorder patients with or without agoraphobia was measured by multi-channel near-infrared spectroscopy (NIRS). [Oxy-Hb] changes in the left inferior prefrontal cortex were significantly associated with the frequency of panic attacks, and, in addition, [deoxy-Hb] changes in the anterior area of the right prefrontal cortex were significantly associated with the severity of agoraphobia. These results suggest that the prefrontal function in patients with panic disorder is associated with the disease state of disease in patients with panic disorder.