Pulmonary hypertension (PH) is defined as a group of diseases characterised by an elevated mean pulmonary artery pressure (Ppa) ≥25 mmHg at rest. Recently, chronic myeloproliferative diseases (CMPD) associated with pulmonary hypertension were included in the group 5 category, corresponding to PH for which the aetiology is unclear and/or multifactorial. CMPD include chronic myelogenous leukaemia, chronic neutrophilic leukaemia and chronic eosinophilic leukaemia (which primarily express a myeloid phenotype and polycythaemia vera), idiopathic myelofibrosis, and essential thrombocytosis in which erythroid or megakaryocytic hyperplasia predominates.