Addison's disease is a primary chronic adrenocortical insufficiency which results in decrease and discontinuation of secretion of mineralocorticoids, glucocorticoids, and cortical androgens. It is a relatively rare disease and the most common cause is autoimmunity. The exact pathogenesis is not completely clear, but it is considered that autoantibodies against enzyme 21-hydroxylase have an important role. The disease manifests as weakness, brown hyperpigmentation of the skin, weight loss, nausea, vomiting and hypotension. The diagnosis is based on signs and symptoms, levels of ACTH in blood and ACTH stimulation test. The treatment consists of lifetime replacement of glucocorticoids and mineralocorticoids. The aim of this thesis is to analyse comorbidities in patients with Addison's disease. This retrospective study includes 27 patients who were treated for Addison's disease at the Department of Endocrinology of the Clinical Hospital Centre Zagreb. The data were collected by reviewing the medical documentation which is available through the hospital information system. Most of the patients were women (59.3%) and the mean age at the time of the diagnosis was 40.6 years. Autoimmunity was the most common cause (96.3%). Most of the patients (85.2%) had at least one autoimmune comorbidity, out of which hypothyroidism (77.8%), hypoparathyroidism (18.5%) and vitiligo (11.1%) were the most common. 59.3% of the patients had comorbidities which are not autoimmune by nature, out of which dyslipidaemia (37.1%) and arterial hypertension (33.3%) were the most frequent. This thesis shows that a large part of patients with Addison's disease have associated autoimmune diseases, mostly hypothyroidism, which correlates with other conducted studies.