We have reported a case of alveolar rhabdomyosarcoma with mediastinal involvement occurring in a 15-year-old boy. Computerized tomography revealed an anterior mediastinal mass with internal mammary nodal involvement, which had metastasized from a tumor of the abdominal wall musculature. When making the differential diagnosis of a mediastinal mass with concurrent distant soft tissue and muscle involvement in an adolescent, rhabdomyosarcoma should be considered.