Quantitative and qualitative ultrastructural analyses were made of selected small muscle fibres from muscle biopsies from 23 patients with Duchenne muscular dystrophy (DMD) and 10 with polymyositis. It is argued that the fibres selected were regenerating. The myofibrillar content of the fibres was similar in both samples. In both there was considerable misalignment of the myofibrils but their orientation tended to improve as the content increased. The mitochondrial content was similar in the two samples but there were more ultrastructural abnormalities in the DMD population. The concentration of glycogen was elevated and the sarcoplasmic reticulum (SR) was much more commonly dilated in the DMD patients. Dilated SR is the prenecrotic change seen most frequently in mature muscle fibres in DMD and seems to be the first ultrastructural manifestation of the disease process. Its occurrence in the regenerating fibres suggest that the primary genetic lesion is being recapitulated in successive generations of fibres.