Patients with chronic primary adrenal insufficiency require life-long glucocorticoid- and mineralocorticoid- replacement therapy. Monitoring of treatment is mainly based on clinical parameters and additional measurement of electrolyte status and plasma renin levels. However, obtained values often do not correspond to the patients’ subjective well-being and thus may not fully reflect optimal treatment. The present study investigated sodium content in the calf muscle and the skin obtained via 23Na-MRI in patients with chronic primary AI and healthy controls. Sodium content assessed by 23Na-MRI correlates with further parameters of mineralocorticoid activity and may serve as an objective method to monitor hormone substitution therapy in patients with adrenal insufficiency.