An 85-year-old man with a 2-year history of progressive lower limb weakness and paresthesia was found to have an IgG kappa monoclonal gammopathy of undetermined significance (mgus). Clinical and electrophysiological studies revealed a severe distal bilateral symmetrical polyneuropathy. A sural nerve biopsy showed extensive nerve fibre loss with the deposition of large amounts of amorphous material throughout the endoneurium. Electron microscopy showed the deposits to be composed of microtubular structures which were located diffusely throughout the endoneurium. The deposits were also located within the lumina of the vasa nervorum, some of which were undergoing disintegration and rupture with release of the proteinaceous material into the endoneurium. The regions of the nerve in which they appeared most numerous showed more severe nerve fibre damage than other areas. These microtubular structures were also observed in disintegrating vessels and adjacent endoneurium. On immunohistochemistry they stained with antibody to IgG. Identical deposits were found in the dermis in which there was a leucocytoclastic vasculitis. Located in linear arrays within the axons of myelinated and unmyelinated fibres were highly organised tubular structures resembling immunotactoids. Identification of immunotactoid-like structures within the nerve is unique and may be another mechanism by which monoclonal proteins can induce nerve fibre injury.