Although refinements in microvascular technique have resulted in great success with free tissue transfer, long-term anastomotic patency, particularly venous, is less predictable in patients with a history of intravenous drug abuse or other causes of intimal damage and/or venous hypertension. We report an unusual case in which an arteriovenous fistula was identified intraoperatively as the cause of pulsatile venous backflow despite a normal preoperative angiogram. A flow disturbance at the site of venous anastomosis and subsequent development of a venous thrombosis prompted more proximal venous exposure and the identification of an arteriovenous fistula. Once ligated, pulsatile venous backflow resolved and successful revision of the venous anastomosis was performed. This case illustrates that an anomalous arteriovenous communication must be considered when high-pressure venous backflow is observed intraoperatively, despite no evidence of an arteriovenous fistula on preoperative angiography, because, if identified and ligated, an arteriovenous fistula may represent a treatable cause of venous thrombosis.