PURPOSE: The aim of this study was to describe the dynamic changes of the cervical dural sac and the spinal cord during neck flexion in patients suffering from Hirayama's disease and to present the usefulness of flexion MR study for the diagnosis. MATERIALS AND METHODS: Seven consecutive male patients (age ranging 17-43 years, mean age 23.7 years) with the clinical diagnosis of Hirayama's disease and 5 healthy subjects (aged 25-32 years) for controls had done cervical MRI from January 2001 through June 2002. Cervical MRI was done in neutral and neck flexed positions using 1.5 T system (Sonata, Siemens, Germany) and obtained images were reviewed by two radiologists. We compared the cervical MRI findings of 7 patients with those of 5 healthy controls regarding neck flexion induced changes in the lower cervical segments. RESULTS: Neutral positioned cervical sagittal MR images revealed subtle or mild cord atrophy in only 2 patients. On maximal neck flexion, AP diameter of the cresent posterior epidural space was increased and also cord flattening with anterior shifting of posterior wall of the lower cervical dural canal was noted in all 7 patients. In all 7 cases, the level and side of spinal cord changes corresponded to the clinical phenotype. All control subjects showed neither cord flattening nor widening of posterior epidural space on neck flexion. CONCLUSION: In patients with the clinical diagnosis of Hirayama's disease, MRI scans obtained on maximal neck flexion showed characteristically dynamic flattening of lower cervical cord and widening of posterior epidural space. Therefore, a flexion MR study is needed to prove the diagnosis.