Extramedullary hematopoiesis (EMH) is the process of proliferation of hematopoietic cells outside the medulla of the bones which contain marrow with hematopoietic stem cells. It usually happens in response to inadequate circulating erythrocytes which fail to satisfy the oxygen demand of the body.

EMH may be caused by myelofibrosis or chronic myelogenous leukemia which lead to decreased production or chronic hemolytic anemia like thalassemia or sickle cell anemia in which cell destruction is accelerated.

Intrathoracic EMH manifests most commonly as paraosseous masses (which originate from within the bone) or sometimes as interstitial pulmonary abnormalities or even as pleural masses.