Magnetic resonance imaging (MRI) of the brain was evaluated in 20 patients with motor neuron disease (MND) and in a control group of 11 healthy people. Bilateral increased signal areas of various sizes in the centrum semiovale, corona radiata, internal capsule, pedunculi of midbrain, pons, medulla and even in the frontal lobe, topographically related with the corticospinal tract, were found in 8 out of 20 patients. Three out of 4 patients with progressive bulbar paralysis and 5 out of 11 cases of amyotrophic lateral sclerosis had abnormal MRI. Such MRI abnormalities have neither been found in patients with progressive muscular atrophy nor in controls, suggesting that they may be the hallmark of pyramidal tract degeneration in motor neuron disease.