Dilated cardiomyopathy (DCM) is a heart muscle disorder characterized by cardiac dilatation and impaired systolic function. In an increasing number of all DCM cases a specific etiology can be identified and in the remaining patients DCM is termed idiopathic. There is a wide variation of the clinical presentation in DCM. The majority of patients manifests classical disease, i.e. heart failure due to left (and right) ventricular systolic dysfunction. However, some cases may come to clinical attention because of supraventricular arrhythmias such as sinus node dysfunction, AV-block or atrial fibrillation. Although a multitude of etiologies may be responsible for DCM (e.g. viral, immunological, toxic), the disease is inherited as a single gene disorder in at least 20 to 35% of cases. Most genetic forms of DCM are caused by autosomal dominant gene defects. Six dominant disease loci on chromosomes 1p1-q1, 1q32, 3p22-p25, 6q23, 9q13 und 10q21-q23 have been identified but the corresponding disease genes are not yet known. X-linked DCM without skeletal muscle disease is a rare variety of adult DCM which can be caused by specific mutations in the dystrophin gene on chromosome Xp21.