Early postoperative arrhythmias are a recognized complication of pediatric cardiac surgery. Diagnosis and treatment of early postoperative arrhythmias were prospectively analyzed in 402 consecutive patients aged 1 day to 18 years (mean 29.5 months) who underwent operation between January and December 2005 at our institute. All children were admitted to the intensive care unit, and continuous electrocardiogram monitoring was performed. Risk factors, such as age, weight, Aristotle Basic Score, cardiopulmonary bypass time, aortic crossclamp time, and use of deep hypothermia and circulatory arrest, were compared. Statistical analysis using the Student t test, Mann-Whitney U test, or Fisher exact test was performed. Multivariate stepwise logistic regression was used to assess the risk factors of postoperative arrhythmias. Arrhythmias occurred in 57 of 402 patients (14.2%). The most common types of arrhythmia were junctional ectopic tachycardia (21), supraventricular tachycardia (15), and arteriovenous block (6). Risk factors for arrhythmias, such as lower age (P = .0041*), lower body weight (P = .000001*), higher Aristotle Basic Score (P = .000001*), longer cardiopulmonary bypass time (P = .000001*), aortic crossclamp time (P = .000001*), and use of deep hypothermia and circulatory arrest (P = .0188*), were identified in a univariate analysis. In the multivariate stepwise logistic regression, only higher Aristotle Basic Score was statistically significant (P = .000003*) compared with weight (P = .62) and age (P = .40); in the cardiopulmonary bypass group, only longer aortic crossclamp time was statistically significant (P = .007*). Lower age, lower body weight, higher Aristotle Basic Score, longer cardiopulmonary bypass time, aortic crossclamp time, and use of deep hypothermia and circulatory arrest are the risk factors for postoperative arrhythmias. Junctional ectopic tachycardia and supraventricular tachycardia were the most common postoperative arrhythmias.