Four children with histiocytosis-X had large solid mediastinal masses at diagnosis. Follow-up studies in three showed large air cysts occupying the sites of the original masses; the fourth patient had small air cysts at the periphery of the mass. All patients survived. One patient was explored and the large air cysts were in the mediastinum with adherence to the adjacent lung. Lesser pulmonary involvement was noted in the lungs of three of the four patients. Cavitation within mediastinal masses in histiocytosis-X appears to reflect two processes. The first is necrosis within the masses themselves. The second is involvement of adjacent lung with adherence of lung and pleura to the mediastinum allowing air to fill the mediastinal cavities.