A 31-yr-old woman presented with a severe and rapidly progressive myopathy affecting proximal limbs, neck flexors and respiratory muscles. Muscle biopsy revealed numerous atrophic fibres with marked structural alterations, without inflammatory infiltrate. By electron microscopy, atrophic fibres displayed many rods. A benign monoclonal gammopathy (IgG, lambda chain) was evident in serum. A sarcolemmal deposit of IgG, lambda chain was found by immunostaining. Plasmapheresis and immunosuppressive therapies produced a decrease in paraproteinemia and a partial clinical improvement. This observation is the third to associate monoclonal gammopathy with "late-onset rod myopathy". The pathogenetic role of paraproteinemia remains unclear.