Background: Transforming growth factor-(TGF-a) is a acidic, heat-stable, 50 amino acid polypeptide which is related to cellular proliferation, malignant transformation, and angiogenesis. There are many similarities between keratoacanthoma(KA) and squamous cell carcinoma(SCC). So, it is often difficult to differentiate keratoacanthoma and squamous cell carcinoma, clinically and histologically. Objective : Our purpose was to examine the patterns of TGF-a expression on KA and SCC using immunohistochemical staining and to evaluate the usefulness of the immunohistochemical staining with antihuman TGF-a antibody on differentiation between KA and SCC. Methods : Formalin-fixed, paraffin-embedded specimens that had confirmed to KA(n=12) or SCC(n=10) were performed by immunoperoxidase staining with monoclonal antihuman TGF-a antibody Results : All of KA dmonstrated a diffuse pattern within tumor lobules, but, at the peripheral rim of cells, showed unstained pattern. In contrast to KA, 40% of the SCC had patchy infiltration within tumor lobules, and 60% of the SCC had diffuse pattern which was similar to the pattern found in KA. All of SCC showed well-defined staining at peripheral cells. Conclusion : Immunohistochemical staining with antihuman TGF-a antibody could be used on differentiation between KA and SCC. (Kor J Dermatol 1996;34(2): 227-232)