We compared the electrodiagnostic studies of 40 patients with chronic acquired demyelinative neuropathy and 18 patients with familial demyelinative neuropathy. Patients with acquired neuropathy had differential slowing of conduction velocity when distal latencies were compared with more proximal conduction velocities in the same nerve, when equivalent segments of different nerves were compared, and when dispersion of compound motor action potentials was examined. Conduction block was noted in some patients. Patients with familial disease had uniform conduction slowing of all nerve segments, and conduction block was not seen.