THE Guillain-Barré syndrome (GBS) is an often fatal neurological condition which is due primarily to disease of the peripheral nervous system. The exact nature of the underlying pathological lesion has remained poorly defined partly because light microscopic findings have varied widely, depending on the duration of the disease process. During the first several days, edema and little other change is detectable. Later, infiltrates of mononuclear cells, degeneration of myelin and occasionally of axons appear.1The single ultrastructural study in this condition was limited to peripheral nerve biopsy material and it suggested that GBS was a disorder of the axis cylinder with only secondary changes of myelin.2 The present study details electron microscopic observations on the nervous system in a fatal case of GBS. Findings are presented which establish the fact that GBS is a primary demyelinating disorder with an unusual pattern of myelin destruction previously unrecognized in