Objective: The objectives of this study were (1) to develop a novel magnetization transfer ratio (MTR) MRI assay of the proximal sciatic nerve (SN), which is inaccessible via current tools for assessing peripheral nerves, and (2) to evaluate the resulting MTR values as a potential biomarker of myelin content changes in patients with Charcot-Marie-Tooth (CMT) diseases.Methods: MTR was measured in the SN of patients with CMT type 1A (CMT1A, n 5 10), CMT type 2A (CMT2A, n 5 3), hereditary neuropathy with liability to pressure palsies (n 5 3), and healthy controls (n 5 21). Additional patients without a genetically confirmed subtype (n 5 4), but whose family histories and electrophysiologic tests were consistent with CMT, were also included. The relationship between MTR and clinical neuropathy scores was assessed, and the interscan and inter-rater reliability of MTR was estimated.Results: Mean volumetric MTR values were significantly decreased in the SN of patients with CMT1A (33.8 6 3.3 percent units) and CMT2A (31.5 6 1.9 percent units) relative to controls (37.2 6 2.3 percent units). A significant relationship between MTR and disability scores was also detected (p 5 0.01 for genetically confirmed patients only, p 5 0.04 for all patients). From interscan and inter-rater reliability analyses, proximal nerve MTR values were repeatable at the slicewise and mean volumetric levels. Charcot-Marie-Tooth (CMT) diseases are a group of inherited neuropathies that affect motor and sensory nerves. A majority of CMT phenotypes can be classified as primary demyelination/dysmyelinating (CMT1) or primary axonal (CMT2) neuropathies.1,2 CMT type 1A (CMT1A z 80% of CMT1 cases 3,4 ) arises from duplication of the peripheral myelin protein 22 (PMP22) gene 5,6 and results in dysmyelination and secondary axonal loss. 7 CMT type 2A (CMT2A z 35% of CMT2 cases 8 ) is caused by missense mutations in the gene that encodes for mitofusin 2 9 and leads to primary axonal degeneration. 8 Although the pathologic features of CMT1A/CMT2A are different, length-dependent axonal loss occurs in both and is predictive of outcomes.