A 73-year-old man presented with fatigue and anorexia and underwent FDG PET/CT scan to evaluate a right lung nodule revealed by a chest CT. The FDG PET/CT not only showed that the lung nodule was hypermetabolic but also revealed that there was significantly increased activity in the region of the duodenum. The findings were suspected as primary duodenal malignancy with metastases. However, an upper gastrointestinal tract endoscopic examination with further culture led to a diagnosis of duodenal cryptococcosis.