A 63-year-old man presented with rising carcinoembryonic antigen levels. Endoscopic examination of the stomach and colon had normal findings. Fluorine 18 fluorodeoxyglucose (FDG) PET/CT was performed and showed multiple mesenteric nodules with moderate radiopharmaceutical uptake (Figure, part A). These abnormal foci were suspected to be metastases from gastrointestinal cancer, although no abnormal activity was observed within the gastrointestinal tract at endoscopy. Follow-up with gallium 68 (68Ga) fibroblast-activated protein inhibitor (FAPI) PET/CT was performed to help detect the primary lesion. This examination showed a focus of increased 68Ga FAPI uptake in the gastric wall (Figure, part B) and a greater number of abnormal foci in the mesentery and omentum than that shown with FDG PET/CT (Figure, parts C, D). The patient subsequently underwent CT-guided biopsy of the gastric wall where there was increased 68Ga FAPI uptake (Figure, part E). A histopathologic examination confirmed the diagnosis of gastric adenocarcinoma (Figure, part F). Studies have shown that 68Ga FAPI is a promising PET tracer that could be more sensitive than FDG for some malignancies (1, 2). Gastric adenocarcinoma sometimes demonstrates a low FDG uptake. As the tumor invades the peritoneal tissues, a fibrotic response may occur, resulting in severe fibrosis. Therefore, the sensitivity of PET imaging of fibroblast activation with 68Ga FAPI in the detection of peritoneal carcinomatosis from gastric cancer may be superior to that with FDG. This case demonstrates an example where 68Ga FAPI outperformed FDG in helping identify the primary lesion and peritoneal carcinomatosis from gastric adenocarcinoma.