A 65-year-old woman with dermatomyositis (DM) was admitted because of disorientation, mental dysfunction, and disturbance of consciousness. Prior to admission, she suffered from septic and hypovolemic shock. There was no evidence of active DM on physical examination and laboratory tests. Cerebrospinal fluid examination revealed no signs of meningitis. Because of clinical symptoms and findings on magnetic resonance images, such as the lesion in the splenium of the corpus callosum that was a low-intensity area on T1-weighted images and a high intensity on T2-weighted images; she was diagnosed as Marchiafava-Bignami disease (MBD). She received a combination of vitamin B, vitamin E, vitamin C, and nicotinic acid. Her symptoms improved gradually, and she was discharged at 1.5 months after admission. There has been no report of a case of DM with MBD. This report may provide useful data with regard to the mechanisms of central nervous system (CNS) disorders in patients with DM.