Endometriosis of the sciatic nerve is a rare but important cause of sciatica. Early on, the symptoms may be cyclic and be treated with local excision of the endometrioma from the sciatic nerve. If left untreated, it can result in marked cicatricial change of the sciatic nerve and require more radical surgical treatment. Due to the hemorrhagic nature of endometriomas and their characteristic location, the MR imaging findings can permit a specific diagnosis of sciatic endometriosis to be suggested. Therefore, MR imaging is recommended for evaluation of sciatica if the symptoms vary with the menstrual cycle or if there is a history of endometriosis. Early recognition of this disorder will facilitate appropriate treatment and minimize permanent damage to the sciatic nerve. The first surgically proven case of sciatic endometriosis with preoperative MR imaging is presented.