A 56-year-old woman who received surgery for left breast cancer 10 years previously underwent 18F-fluorodeoxyglucose positron-emission tomography (FDG-PET) for postoperative follow-up. FDG-PET revealed high uptake of tracer in the esophagogastric junction. A slightly elevated, lobular lesion was found in the lower third of the esophagus on upper gastrointestinal endoscopy. An endoscopic biopsy revealed squamous cell carcinoma. We performed thoracoscopic subtotal esophagectomy. Histopathological examination showed a polypoid spindle cell tumor arising from superficial squamous cell carcinoma. Immunohistochemically, the spindle cells were immunopositive for vimentin and AE1/AE3, and a carcinosarcoma of the esophagus was diagnosed. MIB-1 labeling indexes estimated by Ki-67 immunostaining showed that the proliferative rate of the sarcomatous component was markedly higher than that of the carcinomatous component. This is the rare reported case of esophageal carcinosarcoma that showed increased accumulation of tracer on FDG-PET.