Background: Congenital long QT syndrome (cLQTS), characterized by prolonged QTc time, syncope or sudden death caused by torsade de pointes ventricular tachycardia, has an autosomal dominant pattern of inheritance and is caused by mutations in genes coding for the cardiac ion channels conducting the depolarizing sodium current, INa, and the repolarizing delayed rectifier potassium currents, IKr and IKs. Methods: We describe a family in which one family member died suddenly and unexpectedly from what was assumed to be cardiac death, and in which another person with symptoms indicating cLQTS had been controlled for more than 40 years with oral potassium chloride and ammonium chloride. She was screened for mutations in LQTS genes using polymerase chain amplification reaction of genomic DNA followed by single-strand conformation polymorphism analysis. Results: A clinically well-described missense mutation, G306R in KCNQ1, coding for the ion channel conducting the IKs current was found. The mutation causes a moderately dominant reduction in the IKs current. Conclusion: The novel observation is that potassium in cLQTS caused by the KCNQ1 mutation protects against syncope in a life-long perspective. Thus, although the mechanism is not fully elucidated, maintaining a normal potassium homeostasis is of general importance. Hence, securing sufficient intake of potassium and careful control of potassium-losing conditions are important elements in the management of patients with cLQTS as a supplement to established therapeutic strategies.