Histiocytic diseases in dogs are a group of diseases in dogs which may involve the skin, and which can be difficult to differentiate from granulomatous, reactive inflammatory or lymphoproliferative diseases. The clinical presentation and behaviour as well as response to therapy vary greatly among the syndromes. There are at least four well-defined canine histiocytic diseases: 1. Canine cutaneous histiocytoma (derived from specialised epidermic dendritic cells, the Langerhans cells) 2. Reactive histiocytosis (immunohistochemical features show that interstitial/dermal DCs are involved) 2.a. Cutaneous histiocytosis (CH) 2.b. Systemic histiocytosis (SH) 3. Histiocytic sarcoma complex (immunohistochemical features of dendritic cells, possibly interdigitating or perivascular DCs) 3.a. Malignant histiocytosis 3.b. Histiocytic sarcoma Localized histiocytic sarcoma Diffuse histiocytic sarcoma Cutaneous histiocytoma Histiocytoma Histiocytoma is a common, benign, cutaneous neoplasm in dogs. Histiocytomas usually occur as solitary lesions, which spontaneously regress, and seldom recur. They can occur in dogs of all ages, but are more likely in dogs under three years of age. Epidermal invasion by cells of histiocytoma frequently occurs and intra-epidermal nests of histiocytes resemble Pautrier's aggregates, characteristically found in epidermotropic lymphoma (Mycosis Fungoides or MF). Epidermal invasion in histiocytoma, or presence of simultaneous multiple histiocytomas, especially in aged dogs, can appear similar to MF or non-epidermotropic cutaneous lymphoma (NECL). Multiple histiocytomas may look like cutaneous histiocytosis, although morphologically histiocytomas are consistently epidermotropic and commonly epidermally invasive, these are not features of cutaneous histiocytosis. Delayed regression of multiple histiocytomas can occur and lesions can persist for up to 10 months. Metastatic histiocytoma Recently, several cases of histiocytoma were observed in which histiocytes had migrated to draining lymph nodes and obliterated them. Pathologists diagnosed histiocytic sarcoma in these instances and prognosis was reported as poor. In 3 instances regression of these lesions occurred spontaneously within 3–4 weeks. In other instances the metastatic lesions of histiocytoma failed to regress and dogs were euthanized. The disease course in these cases extended over several months. Spread beyond lymph nodes to lung has also been observed in some of these cases. These complications are rare. Langerhans cell histiocytosis The presence of multiple histiocytomas is now a well recognized syndrome. However, there is yet another presentation in which widespread cutaneous lesions histologically identical to histiocytoma are observed. Clinically, the lesions are almost confluent in affected regions. Rapid internal spread is observed and the affected animals have all been euthanized. There is one published account of such a case, and we have