A 35-year-old male presented with a variant of neurocutaneous melanosis with leptomeningeal malignant melanoma. He had three pigmented nevi from birth. He suffered diplopia followed by headache. T₁-weighted magnetic resonance (MR) imaging revealed hydrocephalus and a small linear hyperintense lesion in the right frontal cortex. Several parts of the cortical sulci and the brain surface were slightly enhanced by gadolinium. Ventriculoperitoneal shunting was performed and extensive pigmented leptomeninges were recognized. Open biopsy established the diagnosis of leptomeningeal malignant melanoma. Combined chemoimmunotherapy was repeated every other month with monitoring of the 5-S-cysteinyldopa (5-S-CD) level in the cerebrospinal fluid (CSF). The 5-S-CD level decreased after each treatment, but the basal level steadily increased prior to the next treatment. Two years after the onset, he showed paraplegia caused by an extramedullary mass at the T-6 level. MR imaging showed that melanoma had involved the entire subarachnoid space including the whole spine. He underwent emergent removal of the spinal tumor and showed transient marked improvement. Further intensive chemotherapy was given. However, he died 31 months after the onset of massive proliferation of intracranial leptomeningeal melanoma. Measurement of CSF 5-S-CD levels is valuable for evaluating the therapeutic efficacy and for monitoring the progression of melanoma.