We report the case of a female Japanese infant who was diagnosed with incontinentia pigmenti (IP) on the basis of the clinical and pathological findings of characteristic skin lesions and the detection of deletion in the nuclear factor-kappa B essential modulator gene at Xq28. The patient developed repetitive seizures at the age of 7 months when she was diagnosed with acute disseminated encephalomyelitis (ADEM), an inflammatory demyelinating disease of the central nervous system that often occurs after vaccination or infection. The causative infectious agent in this patient seemed to be Mycoplasma pneumoniae because of the increased titer of its serum antibody and the detection of its DNA in the initial cerebrospinal fluid sample. This patient showed significant improvement on receiving immunosuppressive therapy with corticosteroids. This is the second case report presenting an IP patient susceptible to ADEM, and therefore, ADEM should be considered early in the differential diagnosis of acute neurological illness for IP patients.