A 58-year-old man developed dysarthria followed by a personality change. Subsequently, he developed muscle weakness and atrophy of the left upper and lower limbs, leading to repeated falls when he tried to walk. Neurological examination showed mild dementia, dysarthria, dysphagia, atrophy and fasciculation of the tongue, and muscle weakness and atrophy of all four extremities, particularly on the left side. Deep tendon reflexes were slightly diminished in the upper limbs and slightly exaggerated in the lower limbs without Babinski's sign. Cranial MRI revealed marked atrophy of the medial portions of the temporal lobes, more striking on the right, and T2-weighted imaging revealed symmetrical high-intensity signals from the posterior limbs of the internal capsules to the cerebral peduncles in the midbrain, extending to the pons on the left. 125I-IMP SPECT showed diffuse reduction of RI uptake in the frontal and temporal lobes, which was more marked on the right. We diagnosed this is a case of motor neuron disease with presenile dementia, which Mitsuyama et al. proposed as a new clinical entity, as well as a rare example of bilateral degeneration of the pyramidal tract on cranial MRI.