Studies on magnetic resonance neurography (MRN) in diabetic polyneuropathy (DPN) have found proximal sciatic nerve lesions. The aim of this study was to evaluate the functional relevance of sciatic nerve lesions in DPN, expecting correlations with the impairment of large fiber function. 61 patients with diabetes mellitus type 2 (48 with, 13 without DPN) and 12 controls were enrolled, undergoing MRN, quantitative sensory testing, and electrophysiological examinations. There were differences in mechanical detection (Aβ fibers) and mechanical pain (Aδ fibers), but not in thermal pain and thermal detection clusters (C fibers) between the groups. Lesion load correlated with lower Aα, Aβ, and Aδ fiber, but not C fiber function in all participants. Patients with lower function showed a higher load of nerve lesions than patients with elevated function or no measurable deficit despite apparent DPN. Longer diabetes duration was associated with higher lesion load in patients with DPN, suggesting that nerve lesions in DPN may accumulate over time and become clinically relevant once a critical amount of nerve fascicles is affected. Moreover, MRN is an objective method for determining lower function mainly in medium and large fibers in DPN.