Thick round fibres common in cross sections of muscle biopsies from patients with muscular dystrophy are due to contracted and swollen segments of otherwise normal muscle fibres. This contracture leads to segmental fibre breakdown, which is identical with Zenker's waxy degeneration. In biopsies from 90 patients suspected of neuromuscular disease, segmental contracture was seen in all or nearly all patients with infantile muscular dystrophy, necrotic myopathy or acute alcoholic myopathy. It was present in half of the patients with polymyositis or myotonic dystrophy. In resticted forms of muscular dystrophy it was rare as it was in neurogenic atrophy. In 9 clinically normal patients it was absent. In electron micrographs of the initial stage sarcomeres were moderately shortened, the sarcoplasmic reticulum was distended and the mitochondria were normal. In the plasmalemma holes were found, through which glycogen granules were lost into the interstitial tissue. In later stages myofibrils were overcontracted and homogenized; in large areas the plasmalemma was absent. Based on these findings a hypothesis for the development of waxy degeneration is proposed: locally defects of the plasmamembrane cause segmental contracture, glycogen granules and water soluble enzymes are lost through holes in the plasma membrane, and finally the affected fibre segment becomes necrotic.