Lichen myxedematosus (LM) is an idiopathic cutaneous mucinosis, chronic disorder characterized by the appearance of lichenoid papules, nodules, and/or plaques that are due to dermal mucin deposition with a variable degree of fibrosis in the absence of thyroid disease. Based on the clinical presentation and microscopic characteristics of the lesions, this case appears to represent the canine analogue of a localized LM in man, characterized by skin involvement with no systemic findings, where the inclusion criteria are as follows: 1) papular or nodular/plaque eruption; 2) mucin deposition with variable fibroblast proliferation; and 3) the absence of both monoclonal gammopathy and thyroid disease. A 10-year-old female Poodle was admitted to the Veterinary Medical Teaching Hospital for a six-month duration hindlimb lameness that has become worse when a tissue swelling on the bilateral stifle joint developed over the previous month. Examination revealed the soft tissue swelling alopecic lesions were bilaterally symmetrical and confined to the craniolateral aspect of the stifle joint and tibia. Patellar instability in both directions (medially/laterally) was non-painful on palpation. Radiographs revealed bilateral soft tissue swelling and an abnormal mild distal displacement of the patella without any evidence of degenerative joint disease. Complete blood count and serum chemistry performed were normal. A clear with cell population slightly increased by typical lymphocytes and abundant mucin was the synovial fluid arising from the arthrocentesis. Biopsy of the skin and synovial membrane was taken for histopathologic examination. At the laboratory all tissues were stained with hematoxylin and eosin and after with alcian blue for the demonstration of acid mucopolysaccharides. Sections of lesions were also pretreated with hyaluronidase and stained with alcian blue. Microscopically, there was hyperkeratosis and atrophy of the epidermis, the dermis contained mild infiltration of plasma cells and moderate fibroblast proliferation. The mucin accumulation involved the middle and deep dermis separating and disrupting collagen fibers. The synovial membrane contained mild infiltration of plasma cells, moderate fibroblast proliferation, and mucin depositation. Based on the microscopic findings a diagnosis of mucinosis was made and thyroid function testing was recommended. Serum concentrations of triglycerides, cholesterol, and T4 were normal and the presence of thyroid disease was rejected. The diagnosis of idiopathic cutaneous mucinosis--Lichen myxedematosus (LM) was performed.