We describe a very unusual case of gliomatosis cerebri (GC) with oligodendrocytic differentiation. A 65-year-old man presented with a convulsive seizure. Magnetic resonance (MR) documented diffuse enlargement of cerebral hemispheres, brainstem, and right cerebellar hemisphere. After admission, the patient manifested a progressive deterioration of his neurological condition. A right temporal craniotomy and temporal lobectomy were performed to obtain brain decompression and diagnosis. Pathological findings were those of a GC consisting of neoplastic oligodendrocytes. Oligodendrocytic GC is a very rare pathological condition. Diagnosis of GC is usually made at autopsy. Our case confirms that diagnosis by a combination of MR imaging and brain biopsy.