Rhythm disturbances that develop after pediatric surgery for heart disease significantly increase mortality and morbidity risk. The aim of this study was to determine incidence rates of different arrhythmias and identify risk factors for these disturbances in this patient group. The study involved 580 children in the pediatric cardiovascular intensive care unit who had undergone cardiac surgery between May 2001 and December 2002. Each was followed until discharge. The patient who sustained arrhythmia was recorded. Fifty-one of the patients (8.8%, mean age 1.7+/-2.3 years) developed arrhythmias. Twenty-one (41.1%) had supraventricular tachycardia, 12 (23.5%) had junctional ectopic tachycardia, 10 (19.6%) had complete atrioventricular block, 3 (5.8%) had ventricular arrhythmias, and 5 (9.8%) had atrial fibrillation and atrioventricular dissociation. There was a trend toward higher incidence of arrhythmia (rate, 43.1%) in the 0-6 months age group. The incidence rates of arrhythmia after certain procedures were as follows: 75% after Rastelli operation, 16.7% after total anomalous pulmonary venous return repair, 13.8% after ventricular septal defect repair, 12.8% after the arterial switch operation or arterial switch with ventricular septal defect closure for transposition of the great arteries, 12.5% after atrioventricular septal defect repair, 12.1% after total correction of tetralogy of Fallot, 9.1% after bidirectional cava-pulmonary connection and Fontan procedure, and 6.6% after other miscellaneous procedures. The mean cardiopulmonary bypass time was 105.4+/-54.1 min. At the time of arrhythmia appearance, the mean values for electrolyte (sodium 144+/-5 mEq/L, potassium 3.78+/-0.91 mEq/L, ionized calcium 1.15+/-0.33 mmol/L) and arterial blood gas parameters (pH 7.40+/-0.12 and HCO3 24.7+/-6.3 mmol/L) were all in the normal range. Fifteen (29.4%) of the patients with arrhythmias died and in 7 of these cases, the death was directly linked to resistant arrhythmia. Arrhythmias can be life-threatening especially in the early period after pediatric heart surgery. The incidence of arrhythmia in this series was 8.8%. The results identified type of operation as a major risk factor for arrhythmia after pediatric heart surgery and they also suggest that age may be important as well.