A 22-year-old man with primary mediastinal choriocarcinoma and bilateral lung metastases underwent an FDG PET/CT scan after completing chemotherapy. Serum beta-human chorionic gonadotropin had normalized. PET/CT showed increased FDG uptake in the anterior mediastinal and lung lesions, suspicious for residual disease. After resection of the mediastinal and lung lesions, histopathology revealed necrosis and fibrohistiocytic reaction with no viable tumor. There was discordance between beta-human chorionic gonadotropin (negative) and FDG PET/CT (positive) findings, with PET/CT findings being false positive. Awareness of this potential pitfall of FDG PET/CT is important, and caution should be exercised when using FDG PET/CT to assess residual masses after chemotherapy.