A 40-year-old man visited to our hospital due to progressive right hemiparesis. Magnetic resonance imaging demonstrated a heterogeneous contrast-enhanced lesion in the left basal ganglia with compression of the ventricles. A brain biopsy did not demonstrate central nervous system (CNS) lymphoma, although acute demyelination was observed. Despite the administration of steroids, the lesion increased in size, and the patient died three months after admission. An autopsy disclosed perivascular and parenchymal infiltration of lymphoma cells. An immunohistochemical analysis showed that the lesion was a cytotoxic T-cell lymphoma. This case indicates that the development of primary CNS lymphoma of this immunophenotype may be preceded by demyelination with subsequent rapid progression, thus requiring a careful evaluation and meticulous diagnosis.