The 'hung-up' knee jerk (HUKJ) is a specific, although rarely appreciated, clinical sign of Huntington's Disease. A HUKJ was present in 11 of 31 consecutive patients with Huntington's Disease seen in our practice. The clinical characteristics of these patients are described and an EMG recording of the HUKJ is presented from one patient. The history of the description of the HUKJ and its neurophysiology are briefly reviewed.