Adenocarcinoma arising in a foregut cyst of the mediastinum. A case of malignant transformation in a mediastinal cyst of the esophageal reduplication type is presented. The cyst had been recognized 39 years previously, but remained asymptomatic until sudden growth occurred. It was resected totally from the esophagus and the patient recovered well. A review of the literature showed that malignancy in mediastinal foregut cysts is extremely rare. 