Although a prominent thymus is frequently seen on chest radiographs of the infant and occasionally is the source of concern or confusion, pathologic lesions of the thymus gland are uncommon sources of mediastinal masses in children. Thymus cysts are particularly rare, and because most are asymptomatic, they are usually found incidentally at autopsy or as unsuspected masses on chest radiographs. We report two children, both with idiopathic aplastic anemia, in whom acute respiratory symptoms developed in association with sudden widening of the mediastinal shadow on chest radiographs. Surgical and postmortem evidence of hemorrhage into thymic cysts were found in both patients. These cases and others previously reported emphasize that hemorrhage into thymic cysts may cause acute symptoms, with airway compromise and widening of the mediastinum. Thymic cysts should be added to the list of thymic abnormalities that may occur n patients with bone marrow aplasia.