Twenty-five cases of the congenital, idiopathic long QT syndrome, occasionally diagnosed at birth and persisting after the 4th day of life, were analysed retrospectively to determine the medium-term prognosis and to identify risk factors. The age of the patients varied: 13 neonates including 10 of less than 5 days of age, and 12 children 1 to 12 years old. Eleven patients belonged to families with long QT syndromes. The recruitment also varied: syncope or near syncope (9 cases), bradycardias or tachyarrhythmias (9 cases) or by systematic familial enquiry (7 cases). The corrected QT interval (QTc) was greater than 0.44 s in all patients, and 17 patients had conduction (atrioventricular or intraventricular) defects and/or ventricular arrhythmias (tachycardia, torsades de pointe or fibrillation). All except two older children were prescribed betablocker therapy and in 6 cases of resistant arrhythmias a pacemaker was implanted. There were 6 deaths: 4 deaths occurred in neonates who had the longest QT intervals (greater than 0.65 s) complicated by conduction defects and tachyarrhythmias. The other 2 fatalities were in older children who both died during syncope. The 19 survivors were followed up for 3 months to 16 years: 4 children with the shortest QTc values (0.44 to 0.48 s) have completely recovered; 2 neonates treated for 9 months and then weaned off therapy without any complications and 2 older children who were not treated. The other 15 cases are all under treatment but do well and have had no syncopal episodes during follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)