Among perianal neoplasias, adenomas of perianal glands and of the anal sacs are very common in male dogs, although rarely found in females. Adenocarcinomas of apocrine glands in the anal sac are even more uncommon in females, affecting specially bitches older than 9 years. Fifty to ninety per cent of these carcinomas promote the secretion of a PTH-like peptide, causing a paraneoplastic syndrome (pseudohyperparathyroidism). Metastases frequently occurred by lymphatic route, definitely reaching lungs, spleen, liver and bones. The adenocarcinomas of this type have bad prognosis, with mean patient survival time of 12 to 15 months and local recurrence of 50%. This report describes a case of adenocarcinoma of apocrine glands from the anal sac in a Cocker Spaniel bitch of 9 years old, brought to the Veterinary Hospital of the Methodist University of São Paulo due to the presence of a perianal bulk and symptoms such as polyuria and polydipsia. By clinical examination, it was observed a tumor mass of approximately 10 cm of diameter in the left size of the perianal region, showing firm consistence, regular surface, local adherence and discreet sensibility. There were no apparent metastases in abdominal organs as seen by radiography and ultrasonography. Severe hypercalcaemia (18.3 mEq/l) was found. The animal was submitted to tumor surgical excision and the removed mass presented a yellowish irregular lobulated aspect with firm consistence. Histopathological investigation confirmed the finding of an adenocarcinoma of apocrine glands from the anal sac with grade III of malignancy. The owner did not agree with the use of chemotherapy. One year after the surgical procedure, the animal was still in good health, with no evidence of local recurrence or metastasis.