Fibromatoses, a broad group of fibrous proliferations of similiar microscopic appearance, show intermediate biological behavior between benign fibrous lesion and fibrosarcoma. they are characterized by the infiltrative growth and the tendency toward recurrence, but they never metastasize. The mediastinal fibromatosis has been extremely rarely reported. We experienced a case of anterior mediastinal fibromatosis ingrowing onto the anterior chest wall in 2-year-old female. A radical surgical resection and anterior chest wall reconstruction was performed successfully.