Pedicle muscle flaps that are used to enhance surgical site integrity after thoracic cancer resections may simulate a recurrent mass at postoperative imaging. However, such flaps have a characteristic appearance and location that should allow their differentiation from a solid mass or fluid collection. The location and appearance of a flap depend on the type of muscle used (whether intercostal, serratus anterior, or latissimus dorsi) and the extent of resection. The appearance also varies according to the presence and amount of fat and calcification in the flap. Most flaps have fat strands along the flap axis that increase in size over time. Linear areas of calcification also may occur because of the inclusion of periosteum in a flap, or more extensive calcification may lead to ossification over time. Uncomplicated flaps do not show substantial radionuclide uptake at positron emission tomography (PET), but areas of extensive calcification may show increased uptake similar to that in bone. Vessels also may be seen within a flap, particularly if the flap has a high fat content; and enhancing vessels frequently are evident at contrast material-enhanced computed tomography (CT). The magnetic resonance (MR) imaging characteristics of flaps are similar to those of normal fat and muscle. Although CT is the modality most often used for follow-up imaging, PET or MR imaging may be helpful in cases in which the CT findings are questionable.