Congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency is one of the most common autosomal recessive hereditary diseases. The impairment of cortisol synthesis leads to excessive stimulation of the adrenal glands by adrenocorticotropic hormone (ACTH), adrenal hyperplasia, and excessive androgen synthesis. The syndrome is characterised by a considerable correlation between the genotype and the phenotype with the type of CYP21A2 gene mutation affecting the severity of 21-hydroxylase deficiency. The clinical manifestations of CAH in adults result from adrenocortical and adrenomedullary insufficiency, hyperandrogenism, and the adverse effects of glucocorticosteroids used for the treatment of the condition. Non-classic CAH may sometimes be asymptomatic. In patients with classic CAH obesity, hyperinsulinaemia, insulin resistance, and hyperleptinaemia are more often seen than in the general population. These abnormalities promote the development of metabolic syndrome and its sequelae, including endothelial dysfunction, and cardiovascular disease. Long-term glucocorticosteroid treatment is also a known risk factor for osteoporosis. Patients with CAH require constant monitoring of biochemical parameters (17a-hydroxyprogesterone [17-OHP] and androstenedione), clinical parameters (body mass, waist circumference, blood pressure, glucose, and lipids), and bone mineral density by densitometry. The principal goal of treatment in adults with CAH is to improve quality of life, ensure that they remain fertile, reduce the manifestations of hyperandrogenisation in females, and minimise the adverse effects of glucocorticosteroid treatment. Patients with classic CAH require treatment with glucocorticosteroids and, in cases of salt wasting, also with a mineralocorticosteroid. Radical measures, such as bilateral adrenalectomy, are very rarely needed. Asymptomatic patients with non-classic CAH require monitoring: treatment is not always necessary. Medical care for patients with CAH should be provided by reference centres, as the management of such patients requires collaboration between an endocrinologist, diabetologist, gynaecologist, andrologist, urologist, and psychologist.