© 2022 Wiley Periodicals LLC.Objectives: Fasciculoventricular fiber (FVF) that does not cause tachyarrhythmia is a rare form of ventricular preexcitation, which is important to distinguish from Kent fibers. Although, adenosine and some electrocardiographic features are important in the differentiation of Wolff Parkinson White (WPW) than FVF, a clear distinction may not always be possible without an electrophysiological study (EPS). In this study, we aimed to present the clinical and electrophysiological features of our pediatric patients with fasciculoventricular fiber. Patients and method: Between October 2013 and September 2021, 565 patients who underwent electrophysiological studies due to ventricular preexcitation in our clinic were screened in the study, and 27 (4.7%) patients with fasciculoventricular fiber were included. The data of the patients were obtained from the file records using the electronic internet database system Filemaker®. Electrophysiological study age, weight, gender, symptom, and presence of congenital heart disease of the patients were obtained from the file records. Accessory pathway localization was evaluated according to the modified Arruda algorithm in pre-procedural electrocardiography. In addition, delta wave amplitudes were measured in the first 40 ms from the surface ECG. PR interval, QRS interval, and delta wave amplitude were recorded before and after ablation in patients with additional accessory pathways. Post-procedure values were included in the FVF group. Results: The mean age of the patients was 11.47 ± 4.25 years. All 70.4% of the reasons for admission were symptoms such as palpitations and syncope. Two patients had hypertrophic cardiomyopathy and 1 patient had ccTGA. In the electrophysiological study, additional manifest WPW was found in 9 (33%) patients (3 patients with high risk, 6 patients with orthodromic supraventricular tachycardia), focal atrial tachycardia in a patient, and atrioventricular nodal reentry tachycardia in a patient. While the delta wave amplitude was found to be 2.56 ± 1.38(1–5.5) mm in the first 40 ms in surface electrocardiography in 9 patients with additional accessory pathway, it was found to be 1.64 ± 0.67(0.5–3) mm in the FVF group. There was no statistically significant difference between the 2 groups (p =.398). Delta wave amplitude > 3.5 mm was not detected in any patient with isolated FVF. Interestingly, delta wave amplitude was < 3.5 mm in 7 (78%) of 9 patients who were identified and ablated with an additional accessory pathway. Total 19 of the patients (59.3%) were adenosine-responsive (18 isolated FVF, 1 manifest AP+FVF adenosine-responsive. 8 patients with other manifest AP + FVF had no pre-procedural adenosine-asystole response, and all of them QRS were expanded). Conclusion: Although, the fasciculoventricular fibers themselves are not the cause of tachyarrhythmia, the accessory pathway and other tachyarrhythmia substrate frequency accompanying these cases are quite high (approximately 40%) in EPS. The delta wave characteristics of ablated patients are very similar to FVF patients. While all patients with isolated FVF were adenosine responsive, most of those with additional manifest WPW were unresponsive. Therefore, performing EPS in patients with suspected FVF based on surface ECG features seems to be important for the detection of additional tachyarrhythmias and risky accessory pathways.