As multifocal motor neuropathy (MMN) is a potentially treatable disorder, its differentiation from lower motor neuron disease is important. Evidence of conduction block (CB) is considered one of the relevant criteria for the diagnosis of MMN. Strict criteria for CB may lead to underdiagnosis of MMN, however. Using a standardized examination, we studied the clinical, laboratory, and electrophysiological characteristics of 37 patients presenting with a lower motor neuron disorder and electrophysiological features compatible with segmental demyelination. We propose a set of clinical, laboratory, and electrophysiological criteria for the diagnosis of MMN, which has been verified by follow-up and response to treatment with intravenous immunoglobulins. Based on the clinical, laboratory, and electrodiagnostic features, 21 patients were diagnosed with definite MMN (17 responders), 7 were diagnosed with probable MMN (5 responders), and 9 were diagnosed with possible MMN (1 responder). Age at onset, the number of affected limb regions, and the number of patients with a creatine kinase level greater than 180 U/L were significantly lower in responders than in nonresponders. Elevated anti-GM1 antibodies and definite CB were found significantly more often in responders. The proposed diagnostic criteria may be useful in clinical practice and therapeutic trials.