Conduction block is a common finding in inflammatory neuropathies, indicating circumscribed demyelination. Since demyelination and conduction block are often localized proximally, the whole ulnar nerve including its proximal segments was studied fractionally in 31 patients with inflammatory neuropathies. In 5 of 15 patients with Guillain-Barré syndrome, conduction block at the spinal roots was the first electrophysiological finding indicating demyelinating neuropathy. Segmental conduction blocks were also found in 8 of 9 patients with chronic inflammatory demyelinating polyneuropathies (CIDP) and 4 of 4 patients with atypical neuropathies. In 3 patients, multifocal motor neuropathy (MMN) could only be differentiated from motor neuron disease by recording proximal conduction block. Corresponding to clinical recovery, conduction block improved with immunoglobulin therapy in CIDP and MMN patients. The technique of proximal conduction block studies clearly improves the diagnosis of focal demyelination in inflammatory, treatable neuropathies. They should be performed particularly in patients with atypical neuropathies in whom electrophysiological tests of distal nerve segments show normal results.