A 65 year-old man presented with a 8 months history of ataxia and weakness of the lower limbs. Acroparesthesias had been present for ten years in all four limbs. Serum protein electrophoresis showed a monoclonal IgM kappa spike. Bone marrow showed no increase in plasma cells. Motor conduction velocities were reduced. A biopsy from the right superficial peroneal nerve revealed several enlarged myelin sheaths in each fascicle on semi-thin sections. Under electron microscopy these large myelin sheaths were either regularly arrayed around the axon or formed several large additional loops. The outer lamellae of these abnormal myelin sheaths and of the additional loops were frequently widened. Other fibers showed a widening of some myelin lamellae (W.M.L.) with a myelin sheath of varying thickness, and these widened lamellae were frequently joined together. A direct immunopathological study could not be made. The widening of some myelin lamellae is present in most cases of peripheral neuropathy with IgM monoclonal gammopathy and a fixation of anti IgM serum on some myelin sheaths may be demonstrated by direct immunopathology. Hypermyelination has been reported in a few cases. In this patient hypermyelination was as developed as in typical cases of tomaculous neuropathy. An association in some myelin sheaths of hypermyelination and W.M.L. has been noted in serum induced myelin aberrations in peripheral nervous system cultures.