Mediastinal mature teratomas are rare in children. Diagnosis is often made in the presence of a complication. We report a case of a mature mediastinal teratoma, complicated by its opening in the lung. The patient was a four-year-eight-month-old girl. She was admitted for fever, cough and dyspnea. The chest radiography showed a large mediastinal mass with calcifications. The CT scans and MRI aided in establishing the diagnosis. Biological tests (alpha-foetoproteins and beta-HCG) confirmed the benignity of the tumor. The child was operated on with full success.