We herein report a 75-year-old female patient with intravascular lymphomatosis (IVL) who presented with fever of unknown origin. Examination, including contrast-enhanced CT and (67)Ga scintigraphy, failed to show any lesions. Her blood levels of lactate dehydrogenase and soluble interleukin-2 receptors were high, suggesting a lymphomatous tumor. A bone marrow puncture was negative, and a random skin biopsy revealed a monoclonal proliferation of naked, large lymphocytes in the vascular space of the subcutaneous tissue, confirming the diagnosis of IVL. MRI, performed 7 weeks after admission, showed a brain mass mimicking primary central nervous system lymphoma. The mass was considered to be a collection of malignant lymphocyte cells invading from the vessels. Without the random skin biopsy, this case may have been misdiagnosed as primary central nervous system lymphoma.