A 13-year-old, crossbreed dog presented with a history of recent onset polydipsia, progressive lethargy, weakness and reduced appetite. Blood tests showed raised concentrations of alkaline phosphatase and alanine aminotransferase with marginally low serum potassium. There was a leucocytosis with a mature neutrophilia and no eosinophils. Endocrine tests showed a normal aldosterone concentration and an exaggerated adrenocorticotropic hormone (ACTH) stimulation test, consistent with a diagnosis of hyperadrenocorticism (HAC). A diagnosis of adrenal-dependent HAC was made, based on the presence of a calcified mass involving the left adrenal gland, and hepatomegaly, on radiography and ultrasonography. The owners declined surgical adrenalectomy. Medical management with trilostane rapidly improved the clinical signs and normalised the serum chemistry. ACTH stimulation tests showed an improvement in post-ACTH cortisol concentrations and were used to make dose adjustments where necessary. At the time of writing, no adverse side effects had been seen and the dog remained well after 80 weeks of treatment.