Hypoadrenocorticism is an uncommon but potentially life-threatening condition in dogs. It most commonly arises due to primary failure of all three layers of the adrenal cortex, resulting in reduced cortisol and aldosterone secretion. Clinical findings are non-specific and common to other conditions. Diagnosis can therefore be challenging, since hypoadrenocorticism can mimic other common diseases. A presumptive diagnosis is based on history, clinical signs and laboratory findings. Definitive diagnosis, however, requires an adrenocorticotropic hormone (ACTH) stimulation test. This should be performed especially in patients with a history of vague, waxing and waning gastrointestinal signs, acute collapse or hyponatremia and hyperkalemia.