We examined MR images of the spinal cord from the neutral neck to the fully flexed neck position in Chinese patients with Hirayama's disease (HD). MRI was performed on 25 HD patients and 31 healthy controls in the neutral and fully flexed neck positions. The mean anterior-posterior diameter (APD) and transverse diameter (TD) of the cervical cord at C6 level in the different positions was measured. Localized lower cervical cord atrophy and asymmetric cord flattening in a neutral neck position were highly suggestive of HD. The mean APDs were 5.6+/-0.7 mm in HD patients and 6.7+/-0.5 mm in controls (p <0.001). These changes seemed related to longer disease duration, i.e. >18 months. Although displacement forward of the lower cervical spinal cord without significant compression was observed in healthy young people, anterior shifting of posterior dural sac and lower cervical cord compression in flexion were distinctive and pathognomonic features of HD. The mean APDs were 4.3+/-0.9 mm in HD patients and 6.0+/-0.5 mm in controls in flexion (p <0.001). Our results may be helpful in diagnosing and assessing this disease.