The examination of 40 benign developmental cysts of the mediastinum from the surgical pathology files supports the previous conclusions in the literature about these lesions. In distinguishing bronchogenic from esophageal cysts, the lining epithelium is not helpful, but the presence of two muscle layers in esophageal cysts and bronchial glands or bronchial cartilage in bronchogenic cysts enables categorization in the majority of cases. Gastroenteric cysts are lined by intestinal epithelium and are further distinguished by their location in the posterior mediastinum and association with other developmental anomalies. Pericardial cysts are histologically characterized by their simple, thin-walled structure, and they are usually closely associated with the pericardium and the diaphragm. A small group of cysts, usually bronchogenic or esophageal in type, cannot be readily classified because they are detached and do not contain helpful identifiable structures. An awareness of the developmental anatomy of the tracheo-bronchial tree and esophagus clarifies the relationships in location and structure that are seen in cysts of this region. All the benign developmental cysts in this series were easily removed at surgery, and there were no known postoperative complications.