Patients with Addison's disease (AD) and comorbid type 1 diabetes mellitus (T1DM) are at increased risk of certain acute metabolic disorders relative to patients with one of these conditions only. The reasons for this are unknown.All attendances for acute illness by AD patients at the emergency department of a Sydney hospital between 2000 and 2017 were reviewed. Physiological parameters and illness management strategies were compared between AD patients, those with T1DM and AD combined, and a control group of patients with T1DM.There were 39 presentations for an acute medical illness by 20 non-diabetic AD (28 attendances) and 5 diabetic AD patients (11 presentations) and 40 attendances by 10 T1DM controls. In AD patients, 17 (43.6%) attendances were medically-diagnosed adrenal crises (AC) (63.6% [n=7] in diabetic AD and 35.7% [n=10] in non-diabetic AD). This corresponded to an estimated incidence of 12.5 AC/100 patient years (PY) for diabetic AD patients compared to 4.7 AC/100PY for non-diabetic AD patients (p<0.05). Glucocorticoid stress doses preceded 61.5% (n=24) of all attendances. Patients who used stress doses had more presentations than those who did not (2.0±1.3 vs 1.2±0.5, p=0.01). Diabetic AD patients had a lower mean blood glucose level on presentation (5.6+/-3.9mmol/l) than the T1DM control sample (11.6+/-6.2mmol/l) p<0.001. No T1DM patients had hypoglycaemia in the 3.0-3.9 mmol/l range but 2 (18.2%) of the diabetic AD patients had presenting blood glucose levels in this category, (p<0.05). Hyperglycaemia was more common among T1DM control patients (62.5%, n=26) than diabetic AD patients (18.2%, n=2), p<0.01.AD patients with T1DM have a higher incidence of adrenal crisis (AC) and hypoglycaemia than non-diabetic AD patients, and a lower incidence of hyperglycaemia than those with T1DM alone. This information may be of value in counselling patients with T1DM and AD about AC and hypoglycaemia prevention.