A scheme is presented which divides the process of fibre breakdown in Duchenne muscular dystrophy into 5 sequential stages. In Stage 1 the fibre appears superficially normal although there are subtle changes in the relative volumes of the fibre components. In Stage 2 localised overcontraction of the myofibrils results in excessive stretching of the sarcomeres in other regions of the fibre. This continuing process results in the formation of contraction clumps in which the contractile filaments form an increasingly homogeneous mass. By the final stage the normal structural features of the fibre have completely disappeared and it is invaded by macrophages. It is suggested that the central process of myofibril clumping results from a localised inability of the sarcomeres to relax which implies a defect in the mitochondria, Z-line or sarcoplasmic reticulum. This is discussed in relation to other workers' findings of a decrease in the calcium-accumulating capacity of the sarcoplasmic reticulum in Duchenne muscular dystrophy.