We report a 63-year-old man who had a rare bilateral thalamic glioma. He complained of difficulty with calculations and had mental deterioration. T1-weighted images revealed bilateral thalamic swelling with homogeneous low signal and no contrast enhancement. The tumour, showing decrease of N-acetylaspartate and the presence of lactate on magnetic resonance spectroscopy, was diagnosed as an astrocytoma by stereotactic biopsy.