We have assessed the clinical applicability of a previously described new method for analyzing the distribution of conduction velocities (DCV) in peripheral nerve. Surface-recorded median sensorimotor compound action potentials (CAPs) were found to contain contributions from classes of fibers conducting between 25 and 80 m/sec in the elbow-to-axilla segment. Test-retest correlation coefficients ranged from 0.91 to 0.98. Increasing stimulus intensity recruited additional fiber activity into the CAP with relative preferential activation of faster-conducting fibers at lower stimulus levels. Changes in limb temperature altered the conduction velocity of all fiber classes 4% per degree Celsius. A comparison of fiber diameter distribution and DCVs in a normal sural nerve gave a correlation of 0.70 over the range of 7 to 12 mu. Diabetic patients with minimal or no clinical polyneuropathy showed varying degrees of DCV shift toward the slower conduction velocities, sometimes even in the face of normal conventional nerve conduction studies. These findings indicate that the DCV may be considered the electrophysiological counterpart--not equivalent--of the fiber diameter distribution, with the capacity to distinguish subtle differences in conduction properties of normal and diseased human nerves.