AbstractSecond degree atrioventricular block is uncommon in children. In order to evaluate its outcome, and to find early prognostic factors, we reviewed the history and evolution of 21 children with this arrhythmia discovered on their surface electrocardiogram. Twenty-four-hour monitoring displayed variations in the conduction patterns in almost all children, from long PR interval to complete heart block alternating in the same patient. At follow-up (nine months to 19 years), 13 children (62%) had received implantation of a pacemaker because of progression either to complete atrioventricular block or to severe ventricular bradycardia. Atrioventricular conduction improved on exercise in 11 children, but this did not predict a favorable outcome since four of them required pacing. In contrast, deterioration during sinus acceleration in four patients predicted further aggravation. A supra-Hisian location of the block did not protect against the occurrence of syncope in the two patients who underwent His recordings. Finally, the outcome was not linked to age at diagnosis. Second degree atrioventricular block, therefore, should be considered a serious disease because of its trend to worsen towards complete block and/or severe complications. Close follow-up, including repeated 24-hour monitoring, is the most effective way to unmask such a progression and to avoid its clinical consequences.