We would like to propose floating dural sac sign, which is observed as a hyperintense band or rim around the spinal dural sac on axial T2-weighted images, as a sensitive sign to identify cerebrospinal fluid (CSF) leakage. One hundred patients with orthostatic headache were prospectively registered in 11 hospitals. These patients were examined by brain magnetic resonance (MR) imaging (n = 89), radioisotope cisternography (n = 89), MR myelography (n = 86), axial T2-weighted imaging of the spine (n = 70), and computed tomography myelography (n = 2). In this study, we separately evaluated the imaging findings of intracranial hypotension and spinal CSF leakage. Among 100 patients, 16 patients were diagnosed as having spinal CSF leaks. Of 70 patients examined with axial T2-weighted imaging, 14 patients were diagnosed with spinal CSF leaks, and floating dural sac sign was observed in 17 patients, 13 patients with spinal CSF leaks and 4 without CSF leaks (sensitivity 92.9%, specificity 92.9%). Of 86 patients examined by MR myelography, extradural fluid was observed in only 3 patients (sensitivity 21.4%, specificity 100%). The floating dural sac sign was a sensitive sign that can be used to identify CSF leakage. Spinal axial T2-weighted imaging might be a good screening method for spinal CSF leakage that can help to avoid the need for lumbar puncture.