The air bronchogram classically signals an end-air-space or "alveolar" filling process such as alveolar proteinosis and bronchioloalveolar-cell carcinoma. However, it can also occur in interstitial diseases, possibly leading to compressive atelectasis and causing crowding of tissue around open airways. In addition, such disease processes may encroach on distal airways, producing obstructive pneumonia which may then surround open proximal airways, as in sarcoidosis and lymphoma. In mixed "alveolar" and interstitial processes such as Pneumocystis carinii pneumonia and the late stages of hemosiderosis, "alveolar" filling may mask the interstitial disease.