- Myasthenia gravis is a neurologic disorder characterized by intermittent muscle weakness which improves after anticholinesterase medication. The pathogenesis of myasthenia gravis is associated with production of autoantibodies to nicotinic acetylcholine receptor in the motor end plate. Most patients do not have an underlying neoplasm, but in 10–15% of the cases, a thymoma may be detected. Apart from thymoma, no other tumor type or organ has consistently been associated with myasthenia gravis. We describe an unusual case of myasthenia gravis in a patient with malignant mixed Mullerian tumor of the uterine corpus. Initial histology revealed malignant mixed Mullerian tumor of a predominant carcinomatous element. At that time, there were no symptoms of muscle weakness. Intraabdominal metastases were detected later, concommitantly with symptoms of muscle weakness and the diagnosis of myasthenia gravis. Histology of the metastases disclosed an exclusive mesenchymal element with striated muscle differentiation. To the best of our knowledge this case is the first report of myasthenia gravis in a patient with malignant mixed Mullerian tumor of the uterine corpus. We propose that the mechanism contributing to myasthenia gravis in this patient was closely associated with the evolving histology and with the nature of the tumor, so that antibodies produced to muscle-like epitopes exposed by malignant cells could have cross-reacted with acetylcholine nicotinic receptors and caused myasthenia gravis.