Biopsy specimens of sural nerve from 10 patients with multiple sclerosis (MS), of whom only 1 was severely disabled, were assessed by morphometric techniques and compared to nerves from age-matched controls. The frequency of abnormal teased fibers was increased in MS nerve, with many internodes showing at least a 50% reduction in myelin thickness. Myelinated nerve fiber densities were not significantly different in MS and control nerves. Regression analysis of axonal area on number of myelin lamellae indicated a generalized reduction of myelin lamellae in this disorder. It is suggested that peripheral myelin may be involved in multiple sclerosis.