Two groups of "mdx" mice, totalling 36 animals of both sexes aged between 8 and 30 weeks, have been studied. In the first group of 10 males and 10 females, 8 males (at 8, 12, 20 and 30 weeks) and 4 females (at 12 and 30 weeks) showed severe limb muscle degeneration and inflammation with prominent regeneration and central nucleation of myofibres; fibrosis and fatty infiltration were not a feature. Five males (at 8, 20 and 30 weeks) and 2 females (at 30 weeks) also showed myocardial necrosis and inflammation. In the second group of 8 males and 8 females only 1 mouse, female at 25 weeks, showed similar changes including the myocardial lesion. Three females showed only focal myopathic changes. All the remaining animals in both groups were normal. These findings in terms of the severity and persistence of the myopathy and the myocardial lesion, not hitherto noted in mdx, suggest that it may be of value as an animal model of Duchenne muscular dystrophy (DMD) and/or other human muscle diseases. The variability probably reflects a failure to preserve an homozygous strain.