Background: Microvascular free tissue transfer has become the gold standard for breast reconstruction. While safe and reliable, there are operative complications, with hematomas developing under the free flap among the more common. These can compromise flap viability, lead to hemodynamic instability and infection. This study aims to identify predictors of hematomas following free-flap breast reconstruction. Methods: A prospective study was undertaken of patients undergoing autologous free-flap breast reconstruction over a 4-year period. Precise times to hematoma formation, age, arterial and venous anastomosis time, and anastomosis length were recorded and analyzed for association with time to hematoma formation. Results: One thousand two hundred twelve flaps were undertaken in 1,070 patients during the period of review. Seventy-one (5.8%) flaps were taken back to theater for hematomas. Immediate reconstruction had a significantly higher hematoma rate compared to delayed reconstruction 7.4% versus 5.2% (p <.001). It is noted that there were two main peaks for time to develop hematomas—less than 4 hr postsurgery and between 12 and 15 hr postsurgery. Conclusion: Hematomas are a complication, which must be managed with prompt return to theater to ensure flap salvage and patient stabilization. Predictors for hematoma are presented, with hematomas most likely encountered within the first 12 hr of surgery.