Retinal periphlebitis in multiple sclerosis is of particular interest in relation to our understanding of the pathogenesis of the demyelinating central nervous system plaques. Previous studies have largely been clinical, and there is little detailed histopathological information relating to this condition. We present the first detailed report in the neurological literature on the histological findings in chronic periphlebitis retinae associated with multiple sclerosis. The most significant abnormalities of the affected retinal veins were the presence of thick laminated collagen in the wall, associated with a scanty infiltration of plasma cells.