Members of a family have been investigated because of three sudden deaths among them. Two young sisters, aged 12 and 16, died suddenly while swimming and running, while their 19-year-old brother died suddenly during emotional stress. In no case did autopsies reveal any structural abnormalities. Their 39-year-old mother and her 19-year-old daughter gave a history of syncopes, while having a normal physical examination and normal ECGs. During a treadmill test, multiple ventricular extrasystoles and bursts of polymorphic ventricular tachycardia were provoked. Patient-members of this family have undergone echocardiography, catheterization of the left and right sides of the heart, endomyocardial biopsy, and electrophysiologic studies. A differential diagnosis of an inherited long QT interval syndrome, catecholamine-induced arrhythmias, and arrhythmogenic right ventricular dysplasia have been suggested. Patients were given atenolol and were followed up for 18 months. This therapy has greatly reduced the exertional arrhythmias as assessed by serial treadmill tests.