We compared the clinical, electrophysiological, laboratory, and pathological features of 13 patients with Lewis-Sumner syndrome (LSS) with those of 20 pati ents with multifocal motor neuropathy (MMN). LSS and MMN patients have several c ommon clinical features: age at onset, weakness in the distribution of individua l peripheral nerves, mild wasting, cramps and fasciculations, partial areflexia, and frequent stepwise disease course. Cerebrospinal fluid protein level was nor mal or slightly elevated, but always less than 100 mg/dl. Conduction blocks are the electrophysiological hallmarks of these two neuropathies, and no differences in distribution and number of blocks were found. Contrary to MMN, lower-limb involvement at onset was frequent in LSS but extension to the upper limbs was a frequent later feature of the disease. Cranial nerve involvement was noted in 4 LSS patients during relapses and absent in all MMN patients. The major distingui shing features were the clinical and electrophysiological sensory involvement in LSS, and the lack of anti-GM1 antibodies in LSS, whereas IgM anti-GM1 were found in 40% of MMN patients. Some LSS patients responded to steroid therapy, whereas this was ineffective in MMN. From these features, LSS can be considered an entity distinct from MMN, with its own clinical, laboratory, and electrophysi ological characteristics, and as an intermediate link between chronic inflammato ry demyelinating polyneuropathy and MMN. We compared the clinical, electrophysiological, laboratory, and pathological features of 13 patients with Lewis-Sumner syndrome (LSS) with those of 20 pati ents with multifocal motor neuropathy (MMN). LSS and MMN patients have several c ommon clinical features: age at onset, weakness in the distribution of individua l peripheral nerves, mild wasting, cramps and fasciculations, partial areflexia, and frequent stepwise disease course. Cerebrospinal fluid protein level was nor mal or slightly elevated, but always less than 100 mg / dl. Conduction blocks are the electrophysiological hallmarks of these two neuropathies, and no differences in distribution and number of blocks were found. Contrary to MMN, lower-limb involvement at onset was frequent in LSS but extension to the upper limbs was a frequent later feature of the disease. Cranial nerve involvement was noted in 4 LSS patients during relapses and absent in all MMN patients. The major dingui shing features were the clinical and elect rophysiological sensory involvement in LSS, and the lack of anti-GM1 antibodies in LSS, whereas IgM anti-GM1 were found in 40% of MMN patients. Some LSS patients responded to steroid therapy, and this was ineffective in MMN. From these features, LSS can be considered an entity distinct from MMN, with its own clinical, laboratory, and electrophysi ological characteristics, and as an intermediate link between chronic inflamma ry demyelinating polyneuropathy and MMN.