Abstract. Clinical, gross. and light microscopic findings are described for 36 dogs, 33 females and three males. with adenocarcinomas arising from the apocrine glands of the anal sac. All tumors had light microscopic features of malignancy and 22 of 23 metastasized to iliac and lumbar lymph nodes. Nine dogs had disseminated metastases, but bone metastases were found in only one dog. Differentiated neoplasms formed secretory acini and tubules lined by tall columnar or cuboidal epithelium. Most neoplasms were histologically bimorphic. with glandular areas and solid nests. Parathyroid glands were atrophic. Hypercalcemia (mean = 16.1 mg/dl) was present in 20 of 22 dogs (90%) and hypophosphatemia (mean = 3.2 mg/dl) in 12 of 17 (7 1%). Remission of hypercalcemia by tumor ablation and recurrence of hypercalcemia with tumor regrowth suggested that the tumor produced a substance that caused hypercalcemia. This unique clinicopathologic syndrome is characterized by hypercalcemia in old. predominantly female, dogs with an adenocarcinoma arising from the apocrine glands of the anal sac.Neoplasms occur frequently in the perineum of the dog, with adenomas derived from the circumanal gland the most common [13,32,50]. Other glands in the perineum that may undergo neoplastic transformation include dermal sebaceous and apocrine glands, merocrine anal glands. and the apocrine glands of the anal sac [ 1,8,12,24,25, 27, 281. We describe a tumor that originated from the apocrine glands of the anal sac. In the most complete description of this unique syndrome, only females were reported to have developed this neoplasm, the tumors were malignant, and all dogs had hypercalcemia [40].A syndrome of hypercalcemia associated with malignancy has been seen in dogs [23, 34,401, mice [48], rabbits [49], rats [39]. and infrequently in the horse [26] and cat [6]. Lymphosarcoma [34] is the tumor most frequently associated with hypercalcemia in dogs, although there are isolated reports implicating other neoplasms [ 18, 23, 3 I, 541. Malignant neoplasms with metastasis to bone are the most common cause of hypercalcemia in hospitalized people [29, 531. Many hypercalcemic people, however, have neoplasms without evidence of osteolysis due to bone metastases 454