We present the case of a 56-year-old man suffering from acquired immune deficiency syndrome associated with lymphomatoid granulomatosis. Magnetic resonance imaging performed because of convulsions revealed an ill-defined frontal mass showing hypointense on T1WI and slightly hypointense on T2WI with surrounding edema. An elevated uptake was found on T1-201 single photo emission computed tomography consistent with a central nervous system lymphoma. However, the apparent diffusion coefficient ratio calculated from diffusion-tensor acquisitions was 1.53 and was higher than that of lymphomas in previous reports. Stereotactic brain biopsy was performed and lymphomatoid granulomatosis was confirmed.