Precise classification of distal sensory polyneuropathies (SN) according to fiber type involvement is desirable for clinical and research purposes. The sural sensory response has served as an electrophysiologic gold standard for the assessment of large-fiber sensory dysfunction. However, patients labeled as having small-fiber sensory neuropathies on the basis of a normal sural response frequently have clinical evidence of large-fiber dysfunction. The surface-recorded medial plantar potential has shown promise as a more sensitive indicator of large-fiber sensory dysfunction, but is not widely accepted because of concerns about age effects and a lack of large well-controlled studies in SN. We have thus correlated clinical type of SN: large-fiber sensory neuropathies (LFSN), small-fiber sensory neuropathies (SFSN), and mixed (large- and small-fiber) sensory neuropathies (MFSN) with sural and medial plantar nerve conduction studies in 133 consecutive patients with distal SN and 108 control subjects. A combination of stringent clinical characterization and electrophysiologic features, especially the surface-recorded medial plantar rather than sural potential, was complementary, and permitted a more clear separation of LFSN, MFSN, and SFSN than with either approach used alone.