To investigate the long-term course or natural history of Romano-Ward syndrome, 131 cases from the affected side of 13 families with the syndrome were examined by electrocardiography and a review of the histories. The patients were followed up for 2-13 years, with an average of 7.2 years. Five patients (3.8%) died suddenly, four with QT prolongation and one with a normal QT interval. Fourteen patients (10.7%) with QT prolongation developed episodes of syncope, whereas 58 patients (44.3%) showed only QT prolongation without syncopal episode. Three of the thirteen families were asymptomatic, with no cases of syncopal episodes or sudden death, although more than 50% of the examined cases in these families showed definite QT prolongation. In the other ten families, syncopal attacks and/or sudden death occurred in some of the family members. In two of the ten symptomatic families, the probands developed their first syncopal attacks very late in their lives (at the ages of 64 and 75 years), and both families had been asymptomatic before these elder family members became symptomatic. In two cases, syncopal attacks disappeared after electrocardiographic improvement. The present study suggests that: (a) asymptomatic families of Romano-Ward syndrome exist, (b) asymptomatic families may sometimes become symptomatic, and (c) regression of the manifestations of Romano-Ward syndrome may occur in some cases.