A 48-year-old man with multiple hepatic occupancies was suspected of having an unknown primary tumor. Standardized diagnostic workup, including gastrointestinal endoscopy and subsequent18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET)/computed tomography (CT), failed to identify the primary site. He was enrolled in our study to perform 18F-fibroblast activation protein inhibitor (18F-FAPI) PET/CT for further diagnosis. CT images showed large areas of fatty liver with many hyperdense, well-defined annular and nodular areas in the remaining areas (Fig. 1A). Fused 18F-FDG PET/CT showed intense tracer uptake in the large annular and nodular areas, which are considered malignant (Fig. 1B). However, no high uptake was observed in some small hyperdense foci, which should be differentiated from focal fatty sparing in the background of fatty liver (Fig. 1B). 18F-FAPI PET/CT showed high tracer uptake in all hyperdense lesions, especially the small foci with no abnormal uptake on 18F-FDG images (SUVmax: 23.3 vs. 8.0), corresponding to low-intensity lesions revealed by gadolinium ethoxybenzyl diethylenetria-mine pentaacetic acid (Gd-EBO-DTPA)-enhanced magnetic resonance imaging (Fig. 1C and D). However, no suspected primary tumor was found outside the liver (Fig. 2A and B).