Diffuse interstitial fibrosis of the lungs represents a common clinical, radiologic and pathologic entity. A large number of cases with interstitial fibrosis are designated idiopathic and have been termed cryptogenic fibrosing alveolitis (CFA) (1). Hereditary predisposition, autoimmunity and viral infection have been implicated as etiologic, but their relative importance and mutual interrelations have not been defined. The University of Colorado Medical Center is uniquely suited to a study of these factors by virtue of a fortunate combination of immunologic and virologic expertise and an extremely active pulmonary service with a large number of referral patients. It is the purpose of this study to explore the role of heredity, autoimmunity, viral infections, and their interactions in the etiology and pathogenesis of cryptogenic fibrosing alveolitis. The equipment and techniques necessary for such a study are currently available and in use: Histocompatibility typing, immunofluorescence, viral culture, cell culture, immune electron microscopy.