Neurocutaneous melanosis is caused by postzygotic NRAS mutations in neural crest cells, resulting in large or multiple nevi in the skin and proliferation of leptomeningeal melanocytes in the central nervous system. The onset of neurological symptoms is usually before the age of 2 years, but it can also occur in adults. A 35-year-old male had been asymptomatic for a long time after excision of a large congenital melanocytic nevus, but he developed headache, disturbance of consciousness, and seizure. Methotrexate was ineffective, cerebral pressure was decreased by spinal drainage, and steroid pulse therapy was temporarily effective. Seizures and disturbance of consciousness worsened and the patient died on the 92nd day. Cerebrospinal fluid human melanin black-45 immunostaining and serum 5-S-cysteinyldopa (5-S-CD) were useful in diagnosing melanocytic proliferation, and serum 5-S-CD may be useful in predicting prognosis.