Balloon-assisted maturation (BAM) of arteriovenous fistula (AVF) is a fairly new procedure used to accelerate the process of maturation. As with any procedure, complications do arise. In this retrospective analysis of 336 office-based BAM procedures, 5 major complications were analyzed. These were categorized as formation of wall hematoma, extravasation or rupture, spasm, thrombosis, and formation of puncture-site hematoma. Prospective data were collected from May 14, 2009 to March 3, 2011 on 336 office-based duplex-guided BAM procedures. Access site puncture, vessel cannulation, wire placement, and balloon advancement and insufflation were duplex guided. Balloon calibers were chosen based on duplex vein measurements and surgeon preference (approximately 1-2 mm larger than minimal vein diameter). Vascular injuries were classified based on postprocedural duplex assessment. All patients had follow-up duplex scans within a week after BAM. Of the 336 procedures, the most common injury was formation of wall hematoma (136, 40.5%) followed by extravasation or rupture (32, 9.5%), spasm (26, 7.7%), formation of puncture-site hematoma (13, 3.9%), and thrombosis (5, 1.5%). The injuries were further compared based on balloon size, ranging from small balloon group (3-6 mm) to large balloon group (7-12 mm); entry position, retrograde (n=177) versus antegrade (n=159); and type of fistula; radial-cephalic (n=232), brachial-cephalic (n=64), brachial-basilic (n=34), brachial-brachial (n=4), and ulnar-cephalic (n=2). A significant increase in complications was noted in BAM procedures performed in forearm AVF versus upper arm AVF (67% vs 54%, P=0.02) and in the large balloon group versus small balloon group (72% vs 52%, P<0.001). The data suggest that office-based BAM procedures are safe. Fortunately, major complications are not seen at an alarming rate. While increased complications are seen in BAM procedures performed in the forearm and with larger balloons, except for wall hematoma formation (40.19%), each complication occurs in <10% of the procedures. Further studies to help clarify the nature of these complications and their relationship to fistula maturation are warranted.