Leiomyosarcomas of diaphragm are exceptional. Since 1935 only eight cases have been published. We report a new case of a 18-year-old man who consulted for an abdominal mass. Ultrasonography revealed a kystic and septated mass measuring 157 mm of diameter. Tomodensitometry showed a sus-mesocolic and voluminous left tumoral mass wish was widely necrosed. The mass was displacing the stomach on the right. A widely surgical resection of tumour was performed. Diagnosis of leiomyosarcoma of diaphragm was performed by pathological study. The outcome was marked by tumour recurrence three months later. The patient died after six months. The review of literature show that prognosis of diaphragmatic leiomyosarcomas is poor.