Malignant peripheral nerve sheath tumors (MPNST), also known as malignant schwannomas, are highly malignant lesions that tend to arise in the head and neck region or on the extremities, and only rarely in the retroperitoneum. These tumors are not common in children. Almost 50% of cases are associated with neurofibromatosis. When these neoplasms arise in the retroperitoneum, they usually are difficult to diagnose and to treat. Although various radiologic imaging methods are helpful for identifying some features of the mass, definitive diagnosis requires histologic examination and immunohistochemical staining. After treatment, the tumor recurs in 25% of patients. Five-year survival rates as high as 80% have been reported. Total excision, lack of invasion of surrounding tissues and vessels, and absence of neurofibromatosis, are features associated with better outcome. The authors present the case of a 1-month-old boy who had malignant schwannoma diagnosed in a unique location over the peritoneum of the anterior abdomen. The location and rapid growth of the tumor permitted early identification and total excision. Copyright (C) 2001 by WB. Saunders Company.