Benign tumors and cysts of the esophagus are rare, and malignant transformation of esophageal cysts is even more rare. In this article, we report on the first case to our knowledge of a patient presenting with metastatic adenocarcinoma arising in a congenital foregut cyst of the esophagus. A 37-year-old woman had multiple cysts along the aerodigestive tract and diffuse soft tissue metastases. Endoscopic biopsies of a mediastinal mass revealed an enteric cyst with a spectrum of columnar epithelium ranging from benign through adenomatous to frankly malignant. There was only transient response to radiotherapy and chemotherapy. Only three other cases of carcinoma arising from esophageal duplication foregut cysts have been reported, and none of these were metastatic. Given the rarity of malignant transformation within an esophageal cyst, prophylactic excision of known esophageal cysts is not warranted. However, biopsy of the cyst may be useful to exclude the possibility of malignancy, particularly if the onset of symptoms has been recent.