OBJECTIVE: To identify possible factors associated with primary failure of radiofrequency ablation of accessory pathways or recurrence of accessory pathway conduction. PATIENTS AND METHODS: Radiofrequency ablation of accessory pathways failed in 25 of 243 patients, and recurrence of accessory pathway conduction occurred in an additional 13 patients. Factors possibly related to primary failure and recurrence were analysed. RESULTS: Primary failure and recurrence were less frequent in patients with left sided pathways (7% v 19%; 4% v 24%; P = 0.04). The factors that might relate to primary failure included an unstable catheter position (seven patients), a possible epicardial pathway (six patients), or misdiagnosis of accessory pathway location (two patients). The major factors for recurrence included the stability of the local atrial electrogram < or = 0.5 together with the stability of the local ventricular electrogram < or = 0.8, and prolonged time to pathway conduction block > or = 12 seconds). Thirty one patients underwent repeat ablation which was successful in 28. CONCLUSIONS: Primary failure and recurrence were more frequent in patients with right sided pathways. An unstable catheter position and a possible epicardial pathway location are the main contributing factors for primary failure, while unstable local electrograms and prolonged time to block are independent predictors for recurrence.