On the basis of the conception that, in preganglionic lesions, peripheral sensory nerve fibers should remain intact, the question arises whether evaluation of distal sensory nerve action potentials can be helpful in differentiating between cervical dorsal root and peripheral nerve lesions. Amplitudes of sensory nerve action potentials (SNAP) and corresponding distal sensory conduction velocities (SCV) of the median and ulnar nerves were measured in 194 patients with distal sensory impairment and in 20 control subjects. In dorsal root lesions attributable to degenerative changes of cervical spines and/or discs, SNAP and SCV were within the normal range. In peripheral nerve lesions, located in the brachial plexus, or at the elbow or wrist, diminished SNAP amplitudes and/or slowing of SCV were found. It is concluded that in patients with sensory deficit in the hands recordings of SNAP make a differentiation between lesions of dorsal roots and peripheral nerves possible.