Spontaneous intracranial hypotension (SIH) from CSF leak commonly produces headache. It also may produce sagging brain syndrome (SBS), often with neurocognitive symptoms indistinguishable from behavioral-variant frontotemporal dementia (bvFTD). The authors describe a new clinical sign that appears to be pathognomonic of SBS. We reviewed medical records and brain imaging in patients seen at our 2 centers who presented with SIH, SBS, and bvFTD symptoms. There were 51 patients (12 women, 39 men) with mean age 55.5 years (range, 26-70 years). MRI showed severe brain sagging in all. Thirteen patients displayed repetitive flexion with breath-holding at the time of clinical presentation. Five patients had repetitive flexion with breath-holding, which resolved before presenting for evaluation. Thus, 35.3% (18) of 51 patients with SBS displayed seemingly compulsive repetitive flexion with breath-holding. Compulsive repetitive flexion with breath-holding appears to be pathognomonic of SBS, deserving the acronym CoRFBiS (compulsive repetitive flexion with breath-holding in SBS). CoRFBiS should alert the clinician to SBS with SIH as the proximate cause of the clinical constellation, rather than bvFTD.