OBJECTIVE: Describe the morbidity associated with a rare disease due to an embryological defect. METHODS: Retrospective revision of medical and necropsy reports. Bibliographic research using MEDLINE, LILACS and Index Medicus databases. RESULTS: 1 year-old male patient, admitted with generalized tonic-clonic seizures, evolving to deep coma and death in a few hours. Necropsy showed diffuse leptomeningeal malignant melanoma in brain stem, cerebellum, spinal cord and temporal lobe associated with a giant melanocytic nevus and satellite lesions. CONCLUSIONS: Neurocutaneous melanosis is a rare congenital syndrome characterized by the presence of large and/or multiple melanocytic nevi and pigmented tumors of the leptomeninges. It has a poor prognosis as demonstrated by the present report. It's physiopathology is believed to be due to a migration defect of the cells arising from the primitive neural crest. In these cases, an early diagnosis may improve the survival time.