AIM: To study plasma YKL-40 in patients with atrial fibrillation (AF) treated with radiofrequency (RF) catheter ablation and to assess the predictive role of plasma YKL-40 and its changes after restoration of sinus rhythm (SR). METHODS: Forty-six patients (mean age 55 years, range 31-81) with paroxysmal/persistent AF were treated with RF catheter ablation; Holter monitoring for 14 days was performed before ablation and after 3 months. Recurrent symptomatic AF or atrial tachycardia >10 min was considered failure, and the patients were offered a second ablation session. YKL-40 was determined in plasma samples taken prior to ablation and at follow-up visits up to 12 months after ablation. RESULTS: After a maximum of two ablations, 19 patients (41%) had SR without recurrence of AF after 12 months. The patients with no recurrence of AF had significantly lower baseline plasma levels of YKL-40 prior to ablation compared to patients with recurrence of AF (31 vs. 62 microg/l, P = 0.029). Plasma YKL-40 was not an independent predictor of recurrence of AF after ablation. No significant changes in plasma YKL-40 levels were seen from baseline to follow-up at 12 months. CONCLUSION: In patients with paroxysmal or persistent AF treated with catheter ablation, high plasma YKL-40 before ablation is associated with recurrence of AF.