Ambulatory 24-hour electrocardiographic monitoring with a Holter recording system was performed in 100 patients after repair of tetralogy of Fallot. The incidence and severity of ventricular arrhythmia (VA) were studied relative to operative age, follow-up period after corrective surgery, hemodynamic data, ventricular function and operative method. Significant VA (Lown grade 2 to 4) was detected in 41 patients. Patients with significant VA (group I) were older (11.7 +/- 10.0 years old) at operation than those without VA (group II) (5.5 +/- 5.8 years, p less than 0.001). The follow-up period after operation in group I (9.5 +/- 4.8 years) was significantly (p less than 0.001) longer than that in group II (5.5 +/- 4.3 years). The incidence of elevated right ventricular (RV) systolic pressure was significantly (p less than 0.005) higher in group I (43%) than in group II (7%). RV ejection fraction in group I (48 +/- 6%) was significantly (p less than 0.001) lower than that in group II (56 +/- 5%). The new operative method for tetralogy of Fallot without or with minimal right ventriculotomy was more frequently performed in group II (49%) than in group I (15%, p less than 0.005). Pulmonary regurgitation, RV and left ventricular size, and left ventricular ejection fraction were not related to severity of VA. Thus, serious VA was related to higher age at operation, longer interval after surgery, elevated RV systolic pressure, depressed RV ejection fraction and RV scar.