We recorded the motor evoked potentials (MEPs) from the affected thenar muscles during neck flexion in 4 patients in the early progressive phase of Hirayama disease. The MEP size significantly increased during neck flexion in patients compared with normal controls. In 2 patients who were treated with a neck collar, the intrinsic muscle atrophy gradually recovered together with increased grip power, and the time course of changes in MEP during neck flexion became normal after treatment with a neck collar for 7-16 months. The technique described here would be useful for evaluating the vulnerability of cervical motor neurons and the therapeutic effect of the neck collar in Hirayama disease.