We report a patient with motor neuropathy in whom plasma exchange (PE) was followed by a pronounced clinical worsening with the appearance of conduction blocks in previously clinically unaffected motor nerves, leading to the diagnosis of multifocal motor neuropathy (MMN). This report highlights the different response to therapy of MMN and chronic inflammatory demyelinating polyneuropathy (CIDP) because not only steroids but also PE, which is often effective in CIDP, do not improve and at times may even worsen MMN.