The development of cardiac lesions was studied in xmd dogs aged from 1 day to 6 years. Cardiac lesions were not present in dystrophic dogs up to 3 months of age. Foci of mineralization were first seen at 6.5 months. A 1-year-old dog had foci of myocyte hypercontraction. Linear and anastomosing fibrosis was present in all dogs 1 year of age or older, most prominently and most consistently within the subepicardial region of the left ventricular (LV) free wall, the LV papillary muscles, and the right ventricular (RV) aspect of the septum. Ultrastructurally, endomysial fibrosis, decreased myofibrillar density, and prominence of mitochondria were consistent features. Severe degenerative changes were present in two dogs and included prominent intracytoplasmic myelin figures, lipid droplets, and lipofuscin. Immunocytochemical studies of an affected dog confirmed the absence of dystrophin in LV myocardium. Characteristic late-onset cardiac lesions, similar to those occurring in Duchenne dystrophy, are a consistent feature of canine X-linked muscular dystrophy (CXMD).