Cardiac disease is a well-known complication of myotonic dystrophy, understanding of which has been increased by recent advances in both molecular techniques and cardiological investigations. Conduction disturbances and tachyarrhythmias occur commonly in myotonic dystrophy. These have been shown to have a broad correlation in severity with both neuromuscular disease and the extent of the molecular defect in some, but not all, studies. Clinical evidence of generalised cardiomyopathy is unusual. The rate of progression differs widely between individuals; sudden death may be caused by ventricular arrhythmias or complete heart block, and this can be at an early stage of disease. A familial tendency towards cardiac complications has been shown in some studies. The histopathology is of fibrosis, primarily in the conducting system and sino-atrial node, myocyte hypertrophy and fatty infiltration. Electron microscopy shows prominent I-bands and myofibrillar degeneration. Myotonin protein kinase, the primary product of the myotonic dystrophy gene, may be located at the intercalated discs and have a different isoform in cardiac tissue. The role of other genes or the normal myotonic dystrophy allele in myotonic heart disease has yet to be determined. Suggestions for clinical management include a careful cardiac history and a 12-lead ECG at least every year, with a low threshold for use of 24 h Holter monitoring. Extra care should be taken before, during and after general anaesthetics, which carry a high frequency of cardiorespiratory complications. Finally, myotonic dystrophy should be considered in previously undiagnosed patients presenting to a cardiologist or general physician with suspected arrhythmia or conduction block.