Motor neuropathies associated with electrodiagnostic evidence of motor conduction block often improve after treatment with immunotherapy, but there is less evidence about the responsiveness of lower motor neuron (LMN) syndromes without conduction block. In this study we treated four patients with an asymmetric, predominantly distal LMN syndrome associated with high serum titers of IgM anti-GM1 ganglioside antibodies but without conduction block on electrodiagnostic testing. Treatment courses consisted of five to seven repeated monthly regimens of plasma exchange on 2 consecutive days followed, on day 3, by intravenous cyclophosphamide (1 g/m2). The results of treatment were quantitatively measured using hand-held dynamometry. We found that all four patients showed progressive improvement in strength over the 6 to 24 months following treatment. Improvement was documented by both objective muscle testing and patient reports of increased strength and less fatigability. We conclude that immunotherapy may be followed by useful functional benefit in selected patients with an asymmetric, predominantly distal LMN syndrome associated with high serum titers of IgM anti-GM1 antibodies. Gradual improvement often begins as late as 6 to 9 months after the onset of treatment and may persist for 1 to 2 years, or longer, after immunosuppressive treatment is stopped.