Isolated congenital long QT syndrome is an autosomal dominant disorder characterized by recurrent syncopes, ventricular arrhythmias, or sudden death often accompanied by a prolonged QTc interval on ECG. On the occasion of a pregnancy complicated by an intra-uterine death of a full term baby with prolonged bradycardia a long QT syndrome was diagnosed in the mother. Familial examination revealed a prolonged QTc in her mother, brother and sister, all with positive history of syncopes. DNA linkage analysis was subsequently performed in this family with DNA markers on the short arm of chromosome 11. Four of the children in the family, younger than 5 years, were found to be asymptomatic carriers. Three of them showed a clear QTc prolongation on a 12 lead ECG. Another showed QTc prolongation during Holter monitoring but had a normal basic ECG. Measurement of QTc interval in families affected by the long QT syndrome is helpful but does not always permit an accurate diagnosis. Familial screening with DNA linkage analysis especially in families where a member is affected by the syndrome, can reveal "masked" cases which can further be investigated with Holter or effort ECG. The identification of locus heterogeneity of the long QT syndrome complicates genetic diagnosis. Only prospective studies in more families with long QT syndrome can show the additional diagnostic and prognostic value of DNA linkage.