We present a rare case of a 55-year-old male who came to our attention due to the pronounced degree of injury to the peripheral nerves and a decrease in the excitability of their distal segments. Patient had a predominantly distal, chronic (5 years duration), slowly progressive, symmetric, predominantly sensory impairment (hypoaesthesia with hyperpathia) with sensory ataxia and mild weakness. Serum immunoelectrophoresis revealed an IgM-kappa monoclonal protein. CSF protein level was elevated at 3.5 g/L. NCS demonstrated a pronounced demyelinating sensorimotor peripheral neuropathy. Median, ulnar and sural sensory responses were not registered. Sympathetic skin response latencies were 1.8 ms (palm) and 2.3 ms (sole). Blink reflex latencies were prolonged up to 64 ms (R1) and 80 ms (R2). Motor NCS showed a pronounced prolongation of the distal CMAP latencies and conduction velocities decrease: median nerve – 61.0 ms and 9.0 m/s, ulnar – 44.0 ms and 10.0 m/s, peroneal – 58.5 ms and 9 m/s, tibial – 74.0 ms and 10 m/s respectively, femoral – 21.5 ms, facial – 34.2 Terminal latency indexes were smaller than 0.25. CMAP amplitudes was significantly reduced. Attention was drawn to the fact that the proximal CMAP area was greater than distal one. The reduction in CMAP area after distal simulation, as compared to proximal stimulation, was calculated as: (proximal CMAP – distal CMAP) × 100%/proximal CMAP. We called this diagnostic criterion the distal nerve excitability block. This criterion for the tibial and the median nerves was 85.5% and 58.3%, respectively.