Lymphomatoid granulomatosis (LYG) in the CNS is an uncommon lymphoproliferative disease with characteristic angiocentric lymphoreticular proliferative and granulomatous lesions exhibiting low-grade malignant potential. Here we report a rare case of CNS-LYG, which disseminated to the lymph node and bone marrow. A 50-year-old man was diagnosed with CNS-LYG based on brain biopsy showing perivascular infiltration of CD3-positive small T-lymphocytes without overt nuclear atypism. Eight months after the initial neurological symptoms, inguinal lymph node swelling was found and histopathologically diagnosed as peripheral T-cell lymphoma. TCRγ-gene rearrangement study using both paraffin-embedded specimens of brain and inguinal lymph node demonstrated an identical clonal band. Considering the clinical course, we concluded lymph node involvement of CNS-LYG, suggesting the malignant potential of CNS-LYG.