Desmoplastic small round cell tumor is a rare poorly differentiated, aggressive neoplasm that most frequently affects the abdominal cavity. The diagnosis is often difficult requiring morphological and immunohistochemical study. The purpose of our study is to clarify the anatomoclinical aspects of this rare tumor and to discuss the approach of the diagnosis and the prognosis. A 18-year old teenager had presented an enormous intra-abdominal mass. The radiologic findings showed a mass for 20 cm in diameter repulsing the liver and the right kidney. A surgical biopsy was performed. Histopathologic examination demonstrated a proliferation of small round cells surrounded by a desmoplastic stroma. Immunohistochemical staining was positive for pancytokeratin (KL1), epithelial membrane antigen (EMA), vimentin, neuron specific enolase (NSE) and desmine; thus, the diagnosis of a desmoplastic small round cell tumor was made. The patient received a first chemotherapy followed by a maximal surgical reduction. A local recurrence and bone metastasis occurred and the patient eventually died 18 months after the diagnosis from hepatic failure. The diagnosis of desmoplastic small round cell tumor may be suspected in young patient presenting an intra-abdominal small round cell tumor. The treatment is multidisciplinary and its prognosis is poor.