A 56 year old man with acute sensory-motor polyneuropathy associated with mycosis fungoides is described. EMG studies showed diffuse signs of muscle denervation. A skin biopsy specimen showed a lymphocyte infiltration in the dermis, composed of mycosis cells characterised by deep invaginations of the nuclear membrane, and small Pautrier's microabscesses in the epidermis. Sural nerve biopsy revealed endoneurial fibrosis, a decreased number of myelinated fibres and acute axonal degeneration.