Autoimmune destruction of the pancreatic beta cells leads to onset of type 1 diabetes (T1D). Although T1D can occur at any age, 85% of patients are diagnosed under the age of 20. Patients usually present with typical features of thirst, polyuria, weight loss and hyperglycaemia, with or without ketosis. In adults, symptoms are similar to those in children but there is a wider differential diagnosis. The presence of certain other autoimmune diseases, in the patient or relatives, is highly suggestive of an autoimmune cause of diabetes.