A pleomorphic undifferentiated tumor primarily located in the retroperitoneum with a phenotype compatible with an extraosseous Ewing tumor/peripheral primitive neuroectodermal tumor (ET/pPNET) pattern and unusual molecular features is described. Immunohistochemically, HBA-71 (CD99/mic2) and several neural markers were intensively expressed together with scattered cells expressing carcinoembryonic antigen (CEA). Short-term culture showed biphasic neuroblastic and epithelioid cell populations, with the latter expressing germ cell markers (CEA, &agr;-fetoprotein, and the &bgr;-subunit of chorionic gonadotrophin). Conventional cytogenetics displayed several chromosomic rearrangements, especially a complex translocation t(17,2,22,13) (q21:q11→q33::q12→q13::q14). These structural abnormalities were confirmed using fluorescence in situ hybridization analysis. Molecular studies revealed EWS-FEV fusion transcripts (exon 7 of the EWS gene and exon 2 of the FEV gene). In addition, a new p53 mutation not previously reported in ET/pPNET involving exon 5 codon 138: GCC to GAC (Ala/Asp) was detected.In our case, we emphasize the presence of atypical features not only from the phenotypic point of view but also at the genetic level as well as the value of detecting such markers in the differential diagnosis with other abdominal pleomorphic tumors.