Canine cutaneous histiocytoma (CCH) is a common, benign neoplasm of the dog. Histiocytomas most commonly occur as solitary lesions that undergo spontaneous regression. The age-specific incidence rate for histiocytomas drops precipitously after 3 years, although histiocytomas occur in dogs of all ages. Langerhans cells (LCs) in humans and dogs express abundant major histocompatibility complex class II molecules and a variety of leukocyte antigens characteristic of dendritic cell differentiation including CD1a, CD1b, CD1c, and CD11c. The immunophenotype of CCH resembled that of cutaneous LCs by virtue of the expression of CD1 molecules (CD1a, -b, and -c), CD11c, and major histocompatibility complex class II. Furthermore, histiocytoma cells had a tropism for epidermis, which was also consistent with an epidermal LC lineage. The expression of adhesion molecules such as CD11b (variable), CD44, CD54 (ICAM-1), and CD49d (VLA-4) in CCH indicated that the infiltrating cells had some of the characteristics of activated LCs, as these molecules are not expressed by normal, resting canine epidermal LCs. CCH did not express Thy-1 or CD4. Thy-1 expression is a characteristic of human and canine dermal dendrocytes, which are perivascular dendritic antigen-presenting cells closely related to epidermal LCs. CD4 expression is prevalent in human LC histiocytosis, and in this respect CCH differed from human LC histiocytosis. Here we demonstrate that CCH is a localized form of self-limiting LC histiocytosis, which predominantly expresses an epidermal LC phenotype. CCH occurs as solitary or, less commonly, as multiple cutaneous nodules or plaques, which rarely may extend beyond the skin to local lymph nodes. Regression of CCH occurs spontaneously in the vast majority of cases in primary and secondary sites, and is mediated by CD8+ alpha beta T cells. The high frequency of CCH within the general canine population offers the potential that the dog may provide an interesting model system to further the understanding of LC proliferative disorders, particularly the self-limiting, cutaneous form of human LC histiocytosis.