Purpose: Background: Primitive Neuroectodermal Tumor is a rare “small round blue cell tumor” first identified in the early 1900's and belongs to the Ewing's sarcoma family of tumors. The tumor arises from long bones or soft tissue. The incidence is 2.9 cases per million and occur primarily before the age of 20 years. Approximately 9% of cases occur in the third decade of life. Patients often present with pain and a palpable mass. Diagnosis usually made using imaging studies, but biopsy specimens are required for definitive diagnosis. The most commonly used biopsy techniques are either open biopsy or an image guided core biopsy. Fine needle aspirates (FNA) have not classically been used due to the small sample size and lack of tissue architecture. Here we report a case of a retroperitoneal primitive neuroectodermal tumor that was diagnosed by EUS guided FNA. Case Presentation: A 35 year-old man was referred with complaints of left sided flank pain and swelling of 2-3 weeks duration and a retroperitoneal mass on a CT scan. Physical examination revealed a large palpable, non-tender left sided abdominal mass. He had no significant past medical or surgical history. Laboratory work-up revealed mild normocytic anemia (Hgb 12.2) and a mildly elevated alpha-fetoprotein (14.6 ng/ml). CT scan of his abdomen and pelvis revealed a 12.8 × 13 × 12.5 cm cystic and solid mass arising from the retroperitoneum and displacing the third and fourth portions of the duodenum. He underwent an EUS which revealed a well circumscribed heterogeneous mass lesion abutting the inferior portion of the stomach. The mass measured 9.6 x 7.4 cm but extended beyond the view of the ultrasound. EUS-FNA of the mass revealed malignant cells consistent with a primitive neuroectodermal tumor (PNET)/ Ewing's sarcoma. He was subsequently started on neo-adjuvant chemotherapy with a 5 drug regimen that includes: Cytoxan®, Adriamycin®, vincristine, etoposide and ifosfamide. Repeat CT scan 1 month after induction chemotherapy showed the mass had decreased significantly in size. Two months after the initial diagnosis, he was taken for a surgical resection with negative surgical margins. Three months after the resection, he was found to have base of skull osseous metastatic disease. Conclusion: Primitive neuroectodermal tumors are a rare tumor that are often diagnosed using open biopsies. This case demonstrates that EUS-FNA is an emerging modality that can be used to obtain adequate tissue samples from lesions outside the GI tract.