Cavernous angiomas or hemangiomas and malignant lymphomas rarely involve the cavernous sinus. We report the case of a 72-year-old man with right circumorbital pain and right oculomotor nerve dysfunction because of a mass in the right cavernous sinus. It was removed via a transsphenoidal approach and histological examination revealed the mass was a cavernous hemangioma containing atypical large B cells in some sinusoidal vessels; no other evidence of lymphoma was detected on (18)F-2-fluoro-2-deoxy-D-glucose positron-emission tomography or bone marrow biopsy. The patient underwent gamma knife radiosurgery (GKS) of the right cavernous sinus, but no systemic chemotherapy was administered. Although good local control was achieved, the patient developed a systemic diffuse large B-cell lymphoma (DLBCL) 1.5 years after the GKS. The atypical lymphocytes of the cavernous hemangioma and biopsied lymph nodes expressed multiple myeloma oncogene-1 protein. This is a rare case of DLBCL occurring within a cavernous sinus that was diagnosed as hemangioma of the cavernous sinus by neuroimaging, surgical findings, and rapid-freezing histological diagnosis. The case indicates a importance of surgical sampling and detailed histopathological analysis.