We experienced a case of primary leiomyosarcoma of the great omentum with von Recklinghausen's disease. A 28-year-old woman was admitted to the hospital because of left hydrothorax and an abdominal tumor which were pointed out when she consulted with another hospital about dyspnea. The patient was operated on for the tumor under a diagnosis of intraabdominal malignant schwanoma because she was associated with von Recklinghausen's disease. At operation, the tumor was found to originate in the great omentum, directly invading the stomach; a total omentectomy and partial gastrectomy were performed. The tumor, measuring 29×24×10cm, and weighting 4800g, was lined by pseudo capsule. There were no malignant cells left the surgical margin. The histopathological diagnosis was made as leiomyosarcoma of the great omentum, that had invaded the stomach. Primary leiomyosarcoma of the great omentum is an extremely rare disease, with 29 cases having been reported in Japan. Since tumors arising in the great omentum often lack in subjective symptoms and there are no characteristic symptoms of the organ, we have difficulty in preoperative diagnosis. In the treatment of the disease, surgical resection including the metastatic foci as well as invaded organs should be directed.