Background: Lichenoid keratosis (LK) is a rather frequent skin lesion that has some histologic features similar to lichen planus (LP). The clinical and histopathologic characteristics of LK and differential tools from LP are not yet fully established. Objective: The purpose of this study was to investigate the clinical and histopathologic characteristics of LK. Methods: A clinical survey was done with 17 patients diagnosed as having LK. We reevaluated biopsy materials of 17 patients diagnosed during the past 10 years at Asan Medical Center, Seoul, Korea. We performed an immunohistochemical staining in 17 cases of LK and 7 cases of LP using 5 antibodies for CD3, CD4, CD8, CD20, and cutaneous lymphocyte-associated antigen (CLA). Standard streptavidin-biotin peroxidase method using the monoclonal antibodies with 3-amino-9-ethyl-carbazole was used. Results: The male/female ratio was 1:1.1. The mean age at diagnosis was 54.9 years. The face was the most commonly affected site, followed by the arm and forearm, dorsum of hand, chest, trunk, abdomen, and leg. The lesions were predominantly solitary (76.5%); 1 patient had 4 lesions; 3 patients (17.6%) had numerous lesions. The lesions ranged in size from 0.4 to 2.0 cm. Histopathologically, all the cases showed characteristic lichenoid infiltrates of lymphocytes, occasional parakeratosis, and apoptotic bodies in the epidermis without nuclear atypia of keratinocytes. LK could be reclassified into 3 patterns by means of histopathologic findings: LP-like (11/17), seborrheic keratosis-like (3/17), and lupus erythematosus-like (3/17). Immunohistochemical studies revealed that infiltrated epidermal and dermal lymphocytes in LK consisted mainly of CD8⁺ T cells and partly CD20⁺ B cells. In LP, epidermal lymphocytes were mainly CD8⁺ T cells and dermal lymphocytes were CD4⁺ or CD8⁺ T cells. Interestingly, CLA was strongly expressed in LP but not expressed in LK. Conclusion: We reclassified LK as follows: LP-like LK, seborrheic keratosis-like LK, and lupus erythematosus-like LK. Immunohistochemical stains for CLA as well as CD4 and CD8 may be valuable tools in the differential diagnosis between LK and LP. (J Am Acad Dermatol 2000;43:511–6.)