Abstract.A cutaneous mass at the base of the retroauricular region of a 4-year-old, female Golden Retriever was examined pathologically. Histologically, the mass formed multiple nodules consisting of a proliferation of large clear cells with abundant cytoplasm. Mitotic figures among the neoplastic cells were very sparse. The large clear cells were intensely positive for cytokeratins (AE1/AE4, cytokeratin 8 and 18) and moderately positive for lysozyme and contained periodic acid-Schiff-positive granules in the cytoplasm. In addition, small flat cells lined the islands of neoplastic large clear cells, and these were strongly positive for alpha-smooth muscle actin and vimentin, and some were positive for cytokeratin (AE1/AE4), suggesting they were myoepithelial cells. No local recurrence or metastasis has been recognized during the 18 months since surgical excision. On the basis of these findings, the present tumor was diagnosed as apocrine sweat gland adenoma, clear cell variant. There have been few previous reports of canine apocrine adenomas showing a clear cell morphology.Key words: Apocrine adenoma; canine; clear cell; sweat gland.Apocrine glands are the major type of sweat gland in dogs, and the distribution of eccrine sweat glands is limited to the footpad. Apocrine sweat gland tumors are rather common in dogs and tend to occur on the head, neck, and limb. The Golden Retriever is a breed that shows a predisposition to such tumors. 6 Approximately 70% of canine apocrine tumors are benign in nature, but malignant ones tend to recur locally and metastasize to regional lymph nodes and the lung. 5 The recent World Health Organization classification 4 categorizes apocrine sweat gland tumors in domestic animals into adenoma/carcinoma, complex or mixed adenoma/carcinoma (or both), and ductal adenoma/carcinoma. However, there is little information regarding clear cell morphology of these tumors.In human sweat gland tumors, clear cell change of the neoplastic cells has been well demonstrated, 3,8 and such morphologic features are found more frequently in eccrine gland tumors, especially nodular hidradenomas (hidradenocarcinomas) and poromas (porocarcinomas). 8 Moreover, similar clear cell change is also recognized in human salivary tumors. 3,8 There has been a previous report of a clear cell variant of canine acinic cell carcinoma originating from a minor salivary gland in the oral cavity. 1 In this previous case, neoplastic foci were well circumscribed and were composed predominantly of sheets of large clear cells with vacuolar cytoplasm. The cytoplasmic vacuoles did not stain with periodic acid-Schiff (PAS), alcian blue, mucicarmine, or oilred. Immunohistochemically, the neoplastic cells were also negative for several epithelial and mesenchymal markers including keratin and alpha-smooth muscle actin (SMA).Recently, Mikaelian et al. 7 proposed a new type of canine tumor known as ''follicular stem cell carcinoma'' characterized by clear cells forming lobules and nests surrounded by a basement membrane. Among 30 dogs e...