Electrophysiological tests were performed in three patients with surgically corrected tetralogy of Fallot (mean age at evaluation 25 years, mean age at surgical correction 4 years) who had had either a cardiac arrest or transient neurological disturbances (presyncope, syncope) associated with ventricular arrhythmias. All three patients had an excellent haemodynamic result from surgery as judged by echocardiography and cardiac catheterisation. Ambulatory electrocardiographic monitoring and stress exercise testing were normal in two patients and showed complex ventricular ectopy in one. During invasive electrophysiological evaluation all three patients had inducible ventricular tachycardia (monomorphic QRS in two patients, cycle lengths 230 and 240 ms; polymorphic QRS in one patient, mean cycle length 200 ms) with adverse haemodynamic effects in all three patients. These findings suggest that rapid ventricular tachycardia with detrimental haemodynamic consequences, similar to that induced during laboratory study, was the basis for the presenting symptoms in each patient. This possibility was confirmed in one patient who had identical QRS morphology during both spontaneous ventricular tachycardia and that induced during the laboratory study. Thus sudden death or symptoms of syncope postoperatively in patients with surgically corrected tetralogy of Fallot appear to be due to rapid ventricular tachycardia, which may occur despite an apparently excellent surgical result.