Treatment outcomes of sensory-motor polyneuropathies associated with anti--myelin-associated glycoprotein (MAG) antibodies have varied even with relatively intensive immunosuppression. We used plasma exchange and cyclophosphamide to treat four patients with anti-MAG antibody-associated polyneuropathies whose symptoms had progressed in the preceding year. Treatment courses consisted of five to seven monthly regimens of plasma exchange on 2 consecutive days followed by intravenous cyclophosphamide (1 g/m2). Effects of treatment were quantitatively measured with hand-held dynamometry. All four patients showed improvement in both strength and sensation in the 5 to 24 months after treatment. We conclude that selected patients with sensory-motor polyneuropathies associated with high-titer serum IgM autoantibodies against MAG may have quantitative and useful functional improvement after immunotherapy. The improvement continues after completion of treatment and may persist for 1 to 2 years or longer.