To determine the prevalence, distribution, and flow characteristics of intraluminal neovascularization in patients undergoing great saphenous vein (GSV) endovenous laser (EVLT) or radiofrequency ablation (RFA).Duplex ultrasound (DU) was performed in patients undergoing EVLT or RFA before, during, and after their procedures. Follow-up included assessment for deep venous thrombosis and obliteration. When new vessels were identified, the source, extent, direction, and location of flow were noted. Flow channel diameters were measured and the resistivity index (RI) was used to characterize the flow patterns.A total of 102 venous ablations were performed of which 46 were RFA, and 56 EVLT. Arterio-venous fistulae (AVF) were found in five patients that were not identified by DU prior to intervention. Involved segments had variable length and multiple channels (mean diameter 2.2mm). No patient had local or systemic symptoms related to the AVF. The mean RI was 0.42, consistent with an AVF. The perivenous arteries feeding the AVF had enhanced flow but a significantly higher RI (0.63, p<0.001).Multiple small vessels were found directly adjacent to the involved vein segments forming small AVF within the obliterated vein. The prevalence of AVF in the ablated GSV was 5%. This process may be responsible for recanalization or recurrence after endovenous ablation procedures.