Cardiomyopathy of unknown cause occurred in three of six siblings. The course of the illness was marked by life threatening supraventricular and ventricular arrhythmias, sinoatrial block, atrioventricular block, and embolism (in one patient). The disease was characterised by right ventricular dilatation. Two of the three patients died aged 32 and 48. No new cases of the disease were found when a further 33 family members from three generations were investigated.