Background: We describe the case of an isolated bone marrow natural killer (NK) cell lymphoma with central nervous system (CNS) involvement mimicking a cerebral infarct. CNS involvement in isolated bone marrow lymphoma has not been reported previously. Case Report: A 49-year-old man with complaints of fever, confusion, and agitation was presented. A bone marrow biopsy was performed to investigate the etiology of bicytopenia which was consistent with NK-cell lymphoma. Brain MRI findings were suggestive of a subacute infarct. Conclusion: Even if pathological signal changes of the brain without contrast enhancement resembling infarct are detected in patients with lymphoma on the magnetic resonance imaging, CNS involvement of the lymphoma should be kept in mind in the differential diagnosis.