• Subacute polyneuropathy that responded to prednisone was the initial symptom in two patients, one of whom was later found to have histiocytic lymphoma and the other, chronic lymphocytic leukemia. Sural nerve biopsy specimens in both showed extensive segmental demyelination. In the first patient, there was invasion of the myelinated axons by macrophage processes, and the later course of the neuropathy appeared to parallel that of the lymphoma. In the second patient, there was diffuse lymphocytic infiltration of the perineurium and endoneurium, lymphocytes were found beneath the basal lamina, and the demyelination was characterized by extensive vesicular degeneration of the myelin sheath. The demyelination appeared to be cell mediated in the first patient. This was probably the mechanism in the second patient as well, but simple mechanical compression by infiltrating leukemic cells was another possible explanation.