Exploratory Multi-centre Trial In Patients With ET Treated With XAGRID®

Inclusion Criteria: 1. Patients must be able to understand and willing to participate in the study, and provide a personally dated and signed written informed consent form. 2. Patients must have a confirmed diagnosis of ET according to the World health Organisation's criteria. 3. ET patients who are uncontrolled, in the Investigator's opinion, by first-line (or previous) cytoreductive treatment for efficacy or tolerance reasons. 4. Patients who have either commenced treatment with anagrelide hydrochloride in the last 7 days or for whom a decision has been documented to commence treatment with anagrelide hydrochloride Exclusion Criteria: 1. Patients for whom treatment with anagrelide hydrochloride is contraindicated, according to the current XAGRID SmPC. 2. Known or suspected intolerance or hypersensitivity to the product, closely related compounds, or any of the stated ingredients. 3. Patients participating in an interventional research study. 4. Patients on combination therapy or for whom there is an intention to treat with other cytoreductive agents e.g., hydroxyurea, interferon. Patients can however use aspirin and other anti-aggregatory products at the Investigator's discretion.