Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is a disease characterized by chronic, profound, disabling, and unexplained fatigue. Although it is hypothesized that inflammation in the CNS is involved in the pathophysiology of CFS/ME, there were no direct evidence of neuroinflammation in patients with CFS/ME. Activation of microglia and/or astrocytes is related to neuroinflammation. Our recent PET study successfully demonstrated that neuroinflammation (activation of microglia and astrocytes) is present in widespread brain regions in patients with CFS/ME, and is associated with the severity of neuropsychological symptoms. Evaluation of neuroinflammation in patients with CFS/ME may be essential for understanding the core pathophysiology, as well as for developing the objective diagnostic criteria and effective medical treatments for CFS/ME. We here describe related pathophysiological findings and topics, and mention the diagnostic and therapeutic attempts through these findings in Japan.