In unusual cases, extensive soft-tissue defects of the upper extremity may be reconstructed with major fillet flaps. We discuss the use of these flaps as derived from the experience in using them on 8 selected patients during a 5-year period at 2 institutions. Etiology of injury for which these flaps are ideal include trauma and tumor resection or reconstruction. Defect locations included the shoulder, combined defects of the shoulder plus neck, arm or chest wall, upper arm, and forearm. The majority of flaps require microsurgical free-tissue transfer. Unlike the classic spare-part, fillet-flap concept, the partial or complete conversion of an anatomically intact arm can be performed, especially in tumor reconstruction. Major fillet flaps represent a valuable option in the armamentarium for reconstruction in the upper extremity with either pedicled or free-tissue transfers involving extensive tissue defects. Copyright © 2002 by the American Society for Surgery of the Hand.