Whether the Lewis-Sumner syndrome (L-SS) is a distinct entity from other types of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP-ot) remains controversial. The clinical/electrophysiological characteristics and long-term outcomes of 45 L-SS and 35 CIDP-ot patients were retrospectively compared. The CIDP-ot group was composed of 11 patients with a typical CIDP, 17 with a pure sensory form, four with a distal form and three with a pure motor form. In the L-SS group, asymmetric (P<0.001) and monomelic involvement (P=0.04) of the upper limbs (P<0.001) was significantly more frequent; paucisymptomatic forms (Overall Neuropathy Limitations Scale≤1) were less frequent (P<0.001); electroneuromyography showed that conduction block in intermediate nerve segments was the main demyelinating feature, with frequent F-wave abnormalities on nerves without conduction block (44%). Long-term prognosis was globally poorer in the L-SS group with more frequent aggravation during treatment (P=0.02), less frequent treatment withdrawal (P=0.03) and longer time to achieve successful withdrawal (39 vs. 15months). Our study suggests that L-SS patients have a less favourable therapeutic response rate and long-term outcomes. Rapid differentiation of L-SS from other forms of CIDP is important in order to anticipate a more complicated disease course management, with from one side the inefficacy or even harmfulness of corticosteroids and from the other side a difficult weaning procedure. A prospective study is necessary to confirm these results.