Descriptions of endometriosis in 18th century monographs and manuscripts are rare and the recorded macroscopic features of endometriosis seldom support this attribution to the described cases. Recently, we became aware of an anonymous German autopsy report from the 18th century. After transcription, the manuscript was assessed by pathologists with historical expertise. This revealed that the patient died because of a malignant tumor, most probably of a gynecological origin. Furthermore, the described ovarian pathologic findings strongly support the diagnosis endometriotic ovarian cyst. Like Giovanni Battista Morgagni (1668–1772) in his landmark publication De Sedibus et Causis Morborum per Anatomen Indagatis (1761), the author correlated the pathological findings at autopsy with the symptoms of the patient. The identity of the patient could, with high probability, be established as being the Countess of Reitzenstein, the wife of a Prussian general major in the army of Friedrich the Great: Karl Erdmann von Reitzenstein (1722–1789).