A 35-year-old man with severe progressive dilating cardiomyopathy and no clinical signs of muscle disease underwent muscular investigations because of markedly increased serum creatine kinase. Muscle biopsy demonstrated Becker type muscular dystrophy with dystrophin of low molecular weight. Genetic analysis showed a deletion spanning from exon 45 to exon 46 in the Xp21 region. Xp21 Becker type muscular dystrophy must be considered in the differential diagnosis of dilating cardiomyopathy.