The cysts that were found in the mediastinum are usually divided into bronchogenic cysts, enteric cysts, esophageal cysts, and nonspecific cysts. We add to the relatively small body of literature that exists on this topic by reporting a case of a Müllerian cyst occurring in the posterior mediastinum of a 60-year-old female, showing diffuse nuclear positivity for estrogen receptor (ER) and PAX-8. The gene discussed is PAX8; the disease is bronchogenic cyst.