The name" chronic diffuse interstitial fibrosis of the lungs" indicates a condition defined in morbid anatomical terms. Literally and without restriction it could be applied properly to a wide range of conditions, some of which would normally be included in other diagnostic categories. For instance, the late stage of sarcoidosis, widespread chronic pulmonary tuberculosis, dust diseases of the lung, and the end-results of some chronic inflammatory changes include an element of interstitial fibrosis; though in many of them the changes in the interstitial tissue are not widespread or predomi-nant. If we exclude cases which can reasonably be included in one of these diagnostic categories we shall be left with a group in which widespread lung changes characterized by the progressive development of fibrosis in the interstitial tissue of the lung cannot be explained as the result of an otherwise recognized pathological process. It is this group which I shall discuss. Though it seems unlikely that all the cases in the group so defined are due to the same cause, experience shows that at least a large proportion of the group is constituted by cases which present a syndrome recognizable with confidence on clinical, radiological, and physiological features.

In order to put certain matters of terminology into perspective, reference must be made first to a group of cases described as" acute interstitial fibrosis of the lungs" by Hamman and Rich (1944). These four cases were all in young adults. They ran&an acute course with fever, the total duration of the disease from onset to death from right ventricular failure ranging from six weeks to six months. In none of them was clubbing of the fingers recorded. Histologically they were characterized by the following changes. In the alveoli there was some oedema, with red blood cells and a few leucocytes and enlarged epithelial cells. Some alveoli were lined bya hyaline membrane. In places there was necrosis of alveolar and bronchiolar walls. There was extensive progressive proliferation of fibrous tissue in the interstitial tissue. In places the alveolar contents were undergoingorganization. In three out of four cases there was an excess of eosinophils in the tnterstitial tissue. Thus, the syndrome described by Hamman and Rich was both pathologically and clinically an acute process.