Erythrocytosis is a phenomenon with life-threatening complications and a broad differential diagnosis. Erythrocytosis is usually secondary to a cardiopulmonary condition leading to a low arterial oxygen tension. A probable diagnosis can often be made on the basis of the history, physical examination, a measurement of the peripheral oxygen saturation, and simple laboratory tests. The differential diagnosis can be narrowed down by a determination of the erythropoietin concentration and the JAK2 mutation. If the erythrocytosis is found to be non-physiological, then reduction of the haematocrit via bloodletting and, depending on the diagnosis, treatment with acetylsalicylic acid are indicated.