We report three patients who presented with acute onset of shoulder and upper arm pain followed within a few days by predominantly distal upper limb weakness. Nerve conduction studies showed severe and unequivocal focal motor conduction block in the forearm and/or upper arm along with slowing of motor conduction and prolonged F wave responses. Only very mild changes in sensory nerve conduction were found. One patient made partial clinical improvement after 17 months, and there was a significant improvement in the degree of motor conduction block and the motor conduction velocities. A second patient remained unchanged after 5 months. Idiopathic brachial neuritis (IBN) typically presents acutely with brachialgia and acute or subacute non-progressive weakness. Multifocal motor conduction block in nerves in the arm or forearm has not been described in patients with IBN. Multifocal motor conduction block restricted to the upper limbs has been described in focal chronic inflammatory demyelinating polyneuropathy (CIDP) and in multifocal motor neuropathy with multifocal motor conduction block (MMNCB). However, both these conditions have hitherto usually been described as largely painless chronic progressive disorders with a subacute onset. Our patients, with features overlapping MMNCB/CIDP and IBN, represent an as yet unreported clinical variant.