A soluble form of the interleukin-2 receptor (sIL-2R) is secreted upon T-cell activation. Increased blood levels of sIL-2R occur in a variety of immunological diseases. Although the biological function of sIL-2R is incompletely understood, both in health and disease, sIL-2R serum measurements are commonly conducted in clinical practice as it may help to facilitate diagnosis of specific immune-mediated diseases, such as haemophagocytic lymphohistiocytosis and sarcoidosis. In these, and in other immune-diseases, sIL-2R levels may be used as a biomarker to monitor/predict disease activity and treatment response. In this review, we will give a brief overview of the biology of the IL-2/IL-2R system and will subsequently discuss the clinical utility of sIL-2R measurement, especially in the context of haemophagocytic lymphohistiocytosis, sarcoidosis, rheumatoid arthritis, systemic lupus erythematosus, juvenile idiopathic arthritis, adult-onset Still’s disease, ANCA-associated vasculitis, and IgG4-related disease.