Background: Transient high-degree atrioventricular (AV) block is a common cause of syncope in patients with bifascicular block (BFB) but the intermittent nature of AV block makes ECG documentation a challenge. A sensitive and safe tool to investigate BFB patients with syncope would be a bradycardia-detecting pacemaker, which provides a possibility of studying the time relation between the index syncopal episode and the development of high-degree AV block. Methods: Twenty-seven patients with BFB and syncope were studied prospectively. All patients received a single-chamber ventricular-based pacemaker with bradycardia-detecting ability. A bradycardia episode was defined as a heart rate of <30 beats/min lasting ≧6 s. Results: During a median follow-up of 60 months, a bradycardia event was detected in 14 patients (52%), of whom 13 also had documented high-degree AV block on ECG. The median time between the syncopal episode and the first pacemaker-detected bradycardia event was 5 months and after an additional median time of 6 months, high-degree AV block was documented on the ECG. In 10 of 13 patients (77%) high-degree AV block was documented within 24 months of the syncopal episode corresponding to an annual incidence of 19% during the first 2 years of follow-up in the study population. Conclusion: In this group of BFB patients a syncopal episode was highly predictive of the development of high-degree AV block within 24 months, justifying pacemaker therapy without prior ECG verification.