Using the latencies of M and F responses, we assessed motor nerve conduction velocity along the entire course of the median and ulnar nerves from the spinal cord to the muscle in 14 patients with the less severe forms of Werdnig-Hoffmann disease. In these forms, the motor nerve conduction velocities were decreased significantly over both proximal (cord-to-elbow) and distal (elbow-to-wrist) segments in both the nerves as compared with normal values; however, the mean motor nerve conduction velocities in the proximal segments were faster than those in the distal segments by about the same amount as the normal controls. These findings indicate that motor conduction abnormalities in Werdnig-Hoffmann disease are diffuse over the entire course of the nerve and appear to eliminate a dying-back process in which the affected axons are severely damaged, beginning with the more distal sites.