Multiple endocrine neoplasia type 1 was diagnosed in a 12-year-old male crossbred dog. Relevant history included polyuria and polydipsia of four months' duration. Physical examination revealed abdominal enlargement, seborrhoea and polypnoea. Diagnostic tests indicated hypercalcaemia, elevated serum alkaline phosphatase and alanine aminotransferase, an exaggerated response to adrenocorticotropic stimulation of the adrenal gland, lack of cortisol suppression with a low dose dexamethasone suppression test and suppression of cortisol secretion with a high dose dexamethasone test. An enlarged right parathyroid gland was removed surgically and confirmed histopathologically to be a parathyroid adenoma. The pituitary-dependent hyperadrenocorticism was treated successfully with mitotane for 14 months before the patient was euthanased for an unrelated problem.