Twenty-three children and young persons with a congenital long QT syndrome were identified; the median age at the time of referral was 10 years (range 4 days to 19 years) and 14 patients (61%) had a family history of the syndrome. Among the 19 patients with symptoms, the initial symptom was syncope in 13 (69%), aborted sudden death in 5 (26%) and near drowning in 1 (5%). There were three deaths during a combined follow-up period of 67 patient-years (average annual mortality rate 4.5%). Patients who did not respond to therapy with a beta-adrenergic blocker and those who died were significantly younger than the remaining patients at the time of diagnosis (p less than or equal to 0.05 for both). Analysis of 44 treadmill exercise tests performed by 16 patients revealed significant prolongation of the median corrected QT (QTc) interval in response to exercise, with maximal prolongation present after 2 min of recovery (median QTc interval 0.52 s versus a baseline value of 0.47 s, p less than 0.001). Characteristic changes in T wave configuration were noted in 8 of 15 patients on at least one occasion during ambulatory Holter electrocardiographic monitoring, including T wave alternation in two patients, both of whom died shortly afterward. It is suggested that the congenital long QT syndrome is associated with a significant mortality rate in childhood despite the use of conventional therapy in symptomatic patients. Ambulatory Holter monitoring and treadmill exercise testing may be helpful, both in confirming the diagnosis of a long QT syndrome and in monitoring the adequacy of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)