Midline malignant reticulosis is a relentlessly progressive, localized, destructive inflammatory process of upper respiratory tract including the nose, nasopharynx, palate, paranasal sinuses, and midface. Histologically, it is characterized by a dense aceumulation of pleomorphic mononuclear cells with an admixture of inflammatory cells. We report a case of midline malignant reticulosis developed in a 64-year-old man, who showed a serosanguineous nasal discharge, perforation of nasal septum, persistent ulceration of hard palate, and disseminated lesions on the abdominal skin, which was confirmed by histopathalogical examination. In addition we studied infiltrated cells with atypical nuclei by immunohistochemical staining with a pannel of monoclonal antibodies. The results revealed that the atypical cells expressed the T-cell marker, but not the macrophage and B-cell marker. (Kor J Dermatol 28(2): 250 264, 1990)