HONEYCOMB lung is a condition in which the terminal bronchioles and first order respiratory bronchioles undergo pronounced dilatation so that the affected lung assumes a cystic appearance (figs. 1 and 2). It is easily distinguished from bronchiolar (" centrilobular ") emphysema in that the cystic spaces are larger and have thicker walls of fibro-elastic tissue. The condition is brought about by fibrosis of the walls of bronchioles with subsequent dilatation of the fibrotic bronchioles and the adjacent segments of the bronchial tree. It may be found in association with any of the diseases that give rise to interstitial fibrosis, including rheumatoid disease, scleroderma, sarcoidosis, the Hamman-Rich syndrome and industrial poisoning with cadmium or beryllium.Honeycomb lung is frequently associated with the development of right ventricular hypertrophy and congestive cardiac failure. We have studied the small pulmonary arteries in cases of honeycomb lung due to various diseases to see whether pulmonary arterial lesions were present and, if so, to determine whether there was a form of pulmonary vascular disease characteristic of the condition irrespective of the nature of the underlying primary disease. We have also attempted to assess the vascular lesions quantitatively in order to relate their severity to the weight of the right ventricle. MATERIALS AND METHODSSeven patients with honeycomb lung were studied. The relevant clinical findings are given in table I; all had radiographic evidence of pulmonary fibrosis.In all but patient 6, the lungs were removed intact from the body and fixed in an inflated state by the formalin-steam method described by Weibel and Vidone (1961). These fixed inflated lungs were cut in the sagittal plane at l a . intervals and the character and distribution of the honeycomb change were studied in the slices. In patient 6 the collapsed lungs removed at necropsy were studied without prior inflation and selected blocks of lung tissue were fixed by immersion in 4 per cent. neutral formaldehyde.In every case several blocks of lung tissue were embedded in paraffin wax and sections were cut at 5p thickness. They were stained with haematoxylin and eosin, and by the Lawson modification of the Weigert-Sheridan method for elastic tissue with a Van Gieson counterstain for collagen and muscle. A systematic microscopic