Gender dysphoria is a condition in which a discrepancy between biological sex and gender identity causes distress. Many female-to-male transsexuals (FTMTS) are uncomfortable with female breasts. Chest wall contouring surgery is effective for obtaining a male-type chest, reducing mental stress, and increasing sexual satisfaction in such cases. At the Okayama University Hospital Gender Center, we have obtained positive results using an algorithm to determine the most appropriate surgical method for chest wall contouring in FTMTS patients. However, serious complications requiring reoperation, such as hematoma, may still occur. Postoperative hematomas were found in 15 (4.18%) of 358 FTMTS patients who underwent chest contouring surgery at our hospital between 2006 and 2018. Postoperative hematoma was examined retrospectively. The median time to the onset of hematoma was 7 (6-12) h after the initial surgery. The main blood vessels causing bleeding were those in the head-side skin flap region where visual confirmation was difficult and the perforator vessels from the pectoralis major muscle. Intraoperative bleeding and the operation time had a significant impact on the onset of postoperative hematoma. This is the first retrospective study that investigated the blood vessels and other factors contributing to postoperative hematoma development after chest wall contouring.