Previous reports on the anatomic discordance between atrial andventricular insertion sites of left free-wall accessory pathways werelimited and their findings were controversial. The purpose of this studywas to explore the fiber orientation and related electrophysiologiccharacteristics of left free-wall accessory pathways. The study populationcomprised 96 consecutive patients with a single left free-wall accessorypathway (33 manifest and 63 concealed pathways), who underwentelectrophysiologic study and radiofrequency catheter ablation using theretrograde ventricular approach. The atrial insertion site of the accessorypathway was defined from the cinefilms as the site with the earliestretrograde atrial activation bracketed on the coronary sinus catheterduring tachycardia, and the ventricular insertion site was defined as thesite where successful ablation of the pathway was achieved. Forty-twopatients (44%) had their atrial insertion sites 5-20 mm (10 ±3 mm) distal to the ventricular insertion sites (proximal excursion), 30(31%) patients had their atrial insertion sites 5-20 mm (12 ±3 mm) proximal to the ventricular insertion sites (distal excursion), and24 (25%) patients had directly aligned atrial and ventricular insertion sites. Retrograde conduction properties, including 1:1 VA conduction and effective refractory period, were significantly poorer inthe pathways with proximal excursion (302 ± 67, 285 ± 61 msrespectively) than in those with distal excursion (264 ± 56, 250± 48 ms respectively) or direct alignment (272 ± 61, 258± 73 ms respectively). Accessory pathways at the more posteriorlocation had a significantly higher incidence of proximal excursion (P= 0.006), and those at the more anterior location had a higherincidence of distal excursion (P = 0.012). In conclusion, a widevariation in fiber orientations and related electrophysiologic characteristics was found in left free-wall accessory pathways. This mayhave important clinical implications for radiofrequency ablation.