Microsurgical free tissue transfer has become standard reconstruction method after head and neck cancer resection. Post-operative bleeding and hematoma formation without proper management may cause wound infection, flap necrosis, and even massive bleeding from ruptured major vessels. Patients and Methods: From January 1997 to December 2001, 2015 patients with head and neck cancers underwent microsurgical free tissue transfer after tumor resection. Retrospective chart review identified 41 consecutive cases complicated with post-operative bleeding or hematoma accumulation requiring neck re-exploration (2.0%). Results: Among these 41 cases, there was only one female. The median age was 56 years old (ranged from 79 to 36 years old). The cancers resection involved tongue (8), buccal (15), lip (4), mouth floor (2), gum (4), retromolar (3), and pharynx (3). Higher incidence of bleeding and hematoma formation was noted in more advanced tongue and buccal cancer patients. In the 8 patients with tongue cancer, 5 were T3 cases, and 3 were T2 cases. In the 15 patients with buccal cancer, 8 were T4 cases, 4 were T3 cases, and 3 were T2 cases. The bleeding was found from flap itself (12), raw surface of reconstruction beds (8), branches of pedicle (8), and vessels other than pedicle (13). The possible etiologies were bleeding tendency due to abnormal liver function (5) or massive intra-operative transfusion(2), vessel clip dislodge(13), inadequate anastomosis (1), infection over deep neck space (7), and suction erosion of vessel(1). Four cases required microsurgical redo of anastomosis. Four cases developed marginal and partial necrosis of flaps and two of them received local flap reconstructions. Wound dehiscence was noted in two cases. Three cases were complicated with infection. Repeated bleeding was noted in two cases. Conclusions: Bleeding from head and neck regions after microsurgical free tissue reconstruction is not uncommon. Good hemostasis and careful application of the clips are most important in prevention of post-operative bleeding. Severe coughing or pain should be avoided postoperatively. Whenever significant bleeding or hematoma in the neck happens, prompt action should be taken to avoid subsequent major complications.