We studied the role of Langerhans' cells in the pathogenesis of atopic dermatitis. First, the skin specimens were obtained by shaving biopsy in lesional and non-lesional skin from the 18 atopic patients. To analyze the presence of IgE+ and CD1a+ cells, immunohistochemical staining of specimens and flow cytometry analysis of the Langerhans' cell-enriched suspension were done. Second, patient's peripheral lymphocytes were co-cultured with Langerhans' cell-enriched suspension for the T lymphocyte proliferation test. Patient's eosinophil coumt, serum total IgE, serum house dust mite specific IgE were measured, and clinical evaluation grade was evaluated to find out the relationship between the T lymphocyte proliferation test and these parameters. Third, we compared the antigen persenting ability of Langerhans' cells using either house dust mite antigen or Candida albicans antigen. The results are as follows. I. The CD1a+ and IgE+ cells were more frequent1y observed in lesional skin than non-lesional skin. The flow cytometry results revealed that CD1a+ and IgE+ cells were more abundant from lesional skin than non-lesional skin. 2. The PI(proliferation index) of the proliferation response of lesional Langerhans' cells was higher than that of non-lesional Langerhans' cells. 3. The PI of the proliferation response of Langerhans' cells of patients with a high blood eosinophil level(%) was significantly higher than that of patients with a low blood eosinophil level. 4. The PI of the proliferation response of patient's Langerhans' cells with house dust mite antigen was significantly higher than that of patient's Langerhans' cells with C. albicans antigen. These results might suggest that the Langerhans' cells activated by a certain allergen existed in the skin of atopic dermatitis patient and they were able to provoke the immune response by activating T lymphocytes.