Polycythaemia may complicate or be the presenting feature of a wide variety of different pathologies. Early diagnosis and treatment of primary polycythaemia will significantly reduce the morbidity and mortality associated with this condition. Patients with a raised packed cell volume are divided into those with a raised red cell mass (absolute polycythaemia), and those with a red cell mass within their normal range (apparent polycythaemia). A standard investigative approach of an absolute polycythaemia enables patients with primary and secondary polycythaemia to be identified, leaving a group termed idiopathic erythrocytosis. There are a number of physiological situations and pathological events associated with idiopathic erythrocytosis and apparent polycythaemia. Careful follow-up of both groups of these patients is essential to identify possible causative mechanisms.