A 36-yr-old woman with a history of epigastric pain and a recent episode of acute pancreatitis was found at surgery to have a cyst in the tail of the pancreas that was lined by ectopic endometrial tissue. This is, to our knowledge, the first recorded case of endometriosis of the pancreas. The clinicopathologic features of this patient and the possible pathogenesis of her most unusual pancreatic lesion are discussed.