Congenital Malignant Ectomesenchymoma Presenting as a Neck Mass in a Newborn.

: Malignant Ectomesenchymoma (MEM) is a rare, aggressive soft tissue neoplasm with both neuroectodermal and mesenchymal differentiation. Congenital cases are extremely uncommon, posing significant diagnostic and therapeutic challenges. : We report a case of a full-term male neonate presenting with a large congenital neck mass and respiratory distress at birth. Imaging revealed a lobulated, heterogeneously enhancing mass in the left submandibular region with a mass effect on the airway. Open biopsy and gross resection on day six of life confirmed MEM with rhabdomyoblastic and neuroectodermal differentiation. Post-surgical staging classified the tumor as Stage I, Clinical Group II. Despite initial chemotherapy with Vincristine, Actinomycin, and Cyclophosphamide (VAC), tumor recurrence was detected at week nine of chemotherapy, necessitating a transition to Vincristine, Irinotecan, and Temozolomide (VIT). : MEM is an extremely rare neoplasm in infants, particularly in congenital presentations. Diagnosis is challenging due to its mixed histopathological features and broad differential diagnosis, including rhabdomyosarcoma, fibrosarcoma, lymphangioma, and neuroblastoma. Management typically involves multimodal therapy, with surgical resection being the mainstay of treatment. Chemotherapy is often tailored to the tumor's most aggressive component, though standardized treatment protocols remain undefined. This case highlights the importance of early recognition and a multidisciplinary approach in managing congenital MEM, as a differential diagnosis of soft tissue masses in infants, particularly in the head and neck region.