Fibre type differentiation was carried out on 20 biopsies from Duchenne Muscular Dystrophy (DMD) sufferers using the acid-preincubated reaction for myofibrillar ATPase. Fibres, classified as either type 1, type 2 or 2C, were counted and their minimum diameters (least fibre axis) measured. Particular attention was paid to the population of small fibres that becomes increasingly prominent with the increasing age of the patient. Type 1 fibres were always predominant in the fibre population as a whole. The numbers of type 2 fibres declined with the increasing age of the patients while the numbers of 2C fibres increased. All fibre types were represented in the population of small fibres and the ratio of the numbers of types 1:2:2C fibres was approximately 1:1:3. Ultrastructural examination of the small fibres showed them to be at varying stages of regeneration and differentiation. The continuous presence of regenerating fibres in DMD while the muscles are wasting implies that while regeneration can be initiated it becomes increasingly constrained or restricted as the disease progresses. The cause of this restriction and whether it is related to the basic genetic lesion is unknown. It is suggested that the accumulation of fibrous connective tissue interferes with growth, either directly, in the formation of pseudomyotendinous junctions, or indirectly, by reducing nutrient exchange with the vascular system.