A 36-year-old man presented with a red nodule on his left shoulder. Histologically, there were variously sized, irregularly shaped nests throughout the dermis partly extending into the subcutaneous tissue. Masses of centrocyte-like cells were situated in the center of the tumor nests and accompanied by adjacent secondary follicle structures. Partial follicular colonization was seen. Massive plasmacytoid cells were located in the papillary dermis and the periphery of the tumor nests. Immunohistochemically, these centrocyte-like cells were positive for CD19 and alkaline phosphatase, and negative for CD5 and CD10. Cytoplasm of the plasmacytoid cells was positive for IgG and λ-light chain, and negative for IgM, IgA, and κ-light chain. Monotypic immunoglobulin staining including light chain restriction was shown. Clonal immunoglobulin heavy chain gene rearrangement by Southern blot analysis was shown in the tumor tissue. Morphologic, immunohistochemical, and molecular studies revealed that this patient had a cutaneous marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue type. Electron beam (total 40 Gy) irradiation was applied. The tumor disappeared completely. Neither local recurrence or metastasis have appeared during 3 years of follow-up. (J Am Acad Dermatol 2003;48:S82-5.)