Diffuse interstitial fibrosis and secondary honey comb transformation of the lungs represent a serious irreversible condition resulting from numerous affections of heterogenous origin. Yielding, finally, a rather uniform scar pattern the courses of lesions afford, at all events, a merely phasic diagnosis of the underlying processes from the roentgenologic point of view. Any opportunities for tracing up etiologic factors and exerting therapeutic influence, if at all, will be restricted to initial stages of the, otherwise, impetuous development. The roentgen finding may give rise to suspicion in various respects, yet its features are, in principle, indifferent as to etiologic conclusions. Hence, the x-ray pattern requires thorough supplementay informations of detailed history, clinical data, laboratory and bioptic results, as well, in order to gain a chance for substantiate diagnosis and successful management. The topics concerning differential diagnosis are discussed with regard to various interstitial diseases of the lung and, especially, to the field of immunbiologic reactions.