Background : Although several cases of primary cutaneous CD3O-positive large cell lymphoma have been reported, immunohistochemical study on the tumor has not been reported in Korea. Objective : Our purpose is to evaluate the clinicopathologic and immunohistochemical findings of primary cutaneous CD3O-positive large cell lymphoma. Methods : An immunohistochemical study was performed on paraffin sections of 8 primary cutaneous CD3O-positive large cell lymphoma using CD3, CD45RO, CD45, CD2O, CD3O, and epithelial membrane antigen(EMA). Results : 1. Primary cutaneous CD3O-positive large cell lymphoma occurred equally on males and females. The average age of the patients was 49. The lesion occurred most commonly as nodules on the extremities. Spontaneous regression occurred in 3 cases. The most common treatment was radiotherapy. 2. Histopathologically, 2 of 8 cases showed epidermal hyperplasia with intraepidermal neutrophils. Epidermotropism was found focally in 1 case. Inflammatory cells were present at the periphery of the lesions in 2 cases, but they were admixed with the large atypical cells in 1 case. Tumor cells invaded the subcutaneous tissue in 4 of 6 cases. 3. Immunohistochemically, the large anaplastic cells showed CD3O positivity. Tumor cells in all cases showed positivity for CD3, CD45RO, and CD45, but negative for CD2O. Tumor cells stained with EMA in 1 of 6 cases. Conclusion : Primary cutaneous CD3O-positive large cell lymphoma showed immunophenotype of T-cell lymphoma. It showed some clinicopathologic features of lymphomatoid papulosis, which suggests that both entities represent 2 ends(malignant and benign) of a spectrum of CD3O-positive lymphoproliferative disorders. Therefore, for the diagnosis of primary cutaneous CD3O-positive large cell lymphoma, all clinicopathologic and immunohistochemical findings should be considered together. (Korean J Dermatol 2003;41(11) : 1471∼1477)