A 66-years-old man suffering from Guillan-Barré syndrome (GBS) died 11 days after the onset of the disease and was autopsied 12 h later. The histopathological preparations showed oedema and demyelination of ventral roots at the different levels of the spinal cord. Only very few macrophages but some lymphocytes were seen. In electron micrographs the Schwann’s cells appeared to have retained their normal fine structural features. The axons appeared also usually quite normal as well as the majority of the myelin sheaths. The abnormal myelin sheaths displayed two kinds of alterations: (1) Vesicular degeneration of the myelin lamellae. (2) Thickening of the myelin sheath into irregularly arranged structures. The acid phosphatase and acid proteinase activities were increased, whereas the peptidase activity was similar as in the control material. Total cholesterol, phospholipids and individual phospholipids had normal values. These findings suggest that GBS is a primary demyelinating disease, in which axonal changes are only secondary.