Introduction: Tracheoesophageal fistulas (TEFs) can be congenital or acquired. Congenital causes are rare and accompanied by tracheal atresia or stenosis. More recently, greater than 50% of acquired TEFs have been caused by malignancies in the mediastinum. We present a unique case in which an acquired TEF was diagnosed in an unconventional way. Case Description/Methods: A 57-year-old male with history of hypertension, alcohol misuse & ongoing tobacco use (40 pack years) presented to primary care with complaints of progressive dysphagia, initially to solids then later to liquids as well, over the course of 2.5 years. He also had a sensation of a “lump” in his throat but denied reflux symptoms. Vitals, physical exam, & blood work at the time did not reveal anything concerning. He was thought to have oropharyngeal dysphagia. Laryngoscopy showed normal pharynx & significant erythema in the arytenoid and esophageal inlet without any mass. He was lost to follow-up due to the SARS-CoV2 pandemic. Several months later, he came to the Emergency Department with complaints of vomiting, productive cough, 20-pound weight loss and fatigue. CT imaging revealed irregular thickness of the upper thoracic esophagus with enlarged subcarinal lymph nodes, and a 1.5-cm pulmonary nodule in the middle lobe. Due to suspicion for esophageal cancer he underwent esophagoscopy with ultrasonography. Patient was found to have a hypoechoic mass with irregular borders in the middle third of the esophagus, involving 100% of the lumen circumference, measuring up to 15 mm in thickness. Evaluation was limited due to obstruction. Biopsies were obtained. The proximal portion of the esophageal mass was noted to be invading adventitia, making it at least T3 lesion. During the procedure minimal CO2 was used for insufflation, but when attempts were made to traverse the obstruction, capnography revealed an end-tidal CO2 estimating 90 mmHg – indicating a high likelihood of TEF. Cardiothoracic surgery recommended esophageal stenting with gastrostomy tube placement. Pathology revealed squamous cell carcinoma. The patient followed up with Oncology for staging and treatment. Discussion: Identification of TEFs can be challenging. Esophagoscopy, the gold standard for diagnosis, may miss small or subtle TEFs even with fluoroscopy. Recent studies have shown potential of using capnography as a novel method in diagnosing TEFs with the benefit of reducing the need for radiation. Our unique case highlights application of such diagnostic modality to identify a TEF.