Immunohistopathologic and biochemical studies ofdifferent collagen types extracted from human scar carcinoma ofthe lungs have been carried out for definition and evaluation ofwhich types ofcollagen are involved in the scarring mechanism of such tumors. Tumor homogenates treated with 0.5 M acetic acid and followed by limited proteolysis with pepsin and then by fractional salt precipitation, demonstrated that Type I collagen consitutes the major collagenous component in addition to a significant increase in Type V collagen extracted from human scar carcinoma of the lung. However, when normal mem-CICATRICIAL, or "scar," cancers were first described by Friedrich in 19391 and Rossle in 19432 as a group of lung cancers that were characteristically originated around peripheral scars in the lung. Since then, there have been numerous attempts to study the relationship between pulmonary scarring and the development of specific types of pulmonary carcinoma. Some of the current concepts suggest that peripheral lung carcinoma can characteristically arise in association with pulmonary scars." 3 4 Others, however, have reported that the carcinoma-related scar is an example of desmoplastic reaction such as is seen in breast, stomach, pancreas, and colon cancers. [5][6][7] It is generally agreed the pulmonary adenocarcinoma has a lower survival rate than squamous cell carcinoma, because the tumor often invades both lymphatic channels and blood vessels at a very early stage in its development7 8 and also produces symptoms later in the course of the disease. Therefore, it becomes worthwhile to investigate the origin and significance of the fibrosing process associated with pulmonary scar carcinoma. The objective of this work is to investigate the different collagen types involved in the scarring process both biochemically ("quantitatively") and immunohistochemically ("qualitatively") for determination of the basic relationship between the scar and the neoplasm. branoalveolar peripheral lung tissues were processed under the same experimental conditions, Type III and IV collagens were relatively higher. Immunohistochemical studies were carried out, and the results confirmed the data above. Furthermore, these studies demonstrated a relative localized increase in Type III collagen in the area surrounding the tumor acini, which suggested that these areas are of active and recent scar formation. This supports the current concept ofthe scar origin as a desmoplastic reaction of the host tissues toward the neoplastic cell growth. (Am J Pathol 1985, 121:322-326)