A 59-year-old man with a 30-year history of multiple recurrences of a giant cell tumor (GCT) of the left knee was referred for an $^{18}F$-FDG PET/CT to evaluate a solitary pulmonary nodule. The nodule was mildly FDG-avid, raising suspicion of malignancy. It was excised and histologically proven to be a GCT pulmonary metastasis. A follow-up PET/CT done 2 years later revealed a new, larger lung mass that was more intensely FDG-avid, but of the same histology. This rare report highlights a pitfall in the evaluation of solitary pulmonary lesions by $^{18}F$-FDG PET/CT in patients with GCT of the bone.