ABSTRACTBackgroundHereditary sensory and autonomic neuropathy type III features marked ataxic gait that progressively worsens over time. We assessed whether proprioceptive disturbances can explain the ataxia.MethodsProprioception at the knee joint was assessed using passive joint angle matching in 18 patients and 14 age‐matched controls; 5 patients with cerebellar ataxia were also studied. Ataxia was quantified using the Brief Ataxia Rating Score, which ranged from 7 to 26 of 30.ResultsNeuropathy patients performed poorly in judging joint position: mean absolute error was 8.7° ± 1.0°, and the range was very wide (2.8°–18.1°); conversely, absolute error was only 2.7° ± 0.3° (1.6°–5.5°) in the controls and 3.0° ± 0.2° (2.1°–3.4°) in the cerebellar patients. This error was positively correlated to the degree of ataxia in the neuropathy patients but not the cerebellar patients.ConclusionsThese results suggest that poor proprioceptive acuity at the knee joint is a major contributor to the ataxic gait associated with hereditary sensory and autonomic neuropathy type III. © 2013 Movement Disorder Society