The etiologies of complete heart block in thirty-one children (mean age 5.5 +/- 5.2 years, range 0-14 years) diagnosed at King Chulalongkorn Memorial Hospital between 1990-2001 were reviewed. Three main groups of patients were identified: 1) patients who presented in utero or in the newborn period (congenital heart block, n = 6), 2) patients who had complete heart block after cardiac surgery (postoperative heart block, n = 10), and 3) children outside the newborn period with a new diagnosis of complete heart block unrelated to cardiac surgery (unknown etiology, n = 15). Among 15 patients in the last group, 5 were asymptomatic (or minimally symptomatic) with complete heart block unexpectedly found. These patients probably had previously undetected congenital heart block. Two patients had complete heart block associated with mild viral illness, but no bradycardia-related symptom. The etiology for heart block in these 2 patients was unknown. Eight patients probably had recent onset heart block because of new bradycardia-related symptoms, or a previously documented normal heart rate. All patients in this group were female (mean age 4.3 +/- 4.3 years, median 3.5 years). All were diagnosed between August and January, and the majority (75%) had a history of non-specific viral illness in the preceding 2 weeks. Seven patients (87.5%) were acutely symptomatic. Syncope and/or seizure were the most common presenting symptoms. Left ventricular systolic dysfunction was found in only one patient. The etiology of complete heart block in these patients probably was an acute viral myocarditis that preferentially affected the conduction system. Two of these eight patients had complete recovery of the atrioventricular conduction. The rest had no improvement or had only partial recovery and subsequently underwent permanent cardiac pacemaker insertion.