An 11-year-old boy presented with acute nausea and right lower quadrant pain. A contrast-enhanced CT demonstrated a 10-cm left retroperitoneal mass and no other suspicious findings. An fluorodeoxyglucose positron emission tomography/computed tomography performed 2 days later demonstrated inflammatory findings in the region of the appendix, hypermetabolic right psoas adenopathy, and the known retroperitoneal mass, which was less hypermetabolic. At laparoscopic surgery, an acute perforated appendicitis, walled-off by omentum, was discovered. Biopsy of the retroperitoneal mass was compatible with ganglioneuroma. This case exemplifies the utility of fluorodeoxyglucose positron emission tomography/computed tomography for detecting an inflammatory or infectious process coincident with a neighboring neoplastic process.