Electrodiagnostic features are particularly useful in distinguishing chronic acquired demyelinating neuropathy from familial demyelinating neuropathy. The electrodiagnostic findings in patients with familial demyelinating neuropathy reveal uniform slowing of NCV (nerve conduction velocity) in all the segments of nerve without conduction block. In contrast acquired group shows nonuniform slowing of NCV with temporal dispersion of CMAP (compound muscle action potential) on proximal stimulation. In addition conduction block is noted in some of acquired patients. We experienced 2 patients from one family with HSMN type I, who showed conduction block and concluded that differentiation between chronic acquired and familial demyelination neuropathy is not possible with conduction block alone in a substantial number of patients.