The purpose of this prospective, longitudinal, noninterventional study is to describe clinical characteristics, evolution of disease burden, and treatment patterns in patients with select subcategories of essential thrombocythemia (ET) or myelofibrosis (MF).

Inclusion Criteria:

• MF cohort: Diagnosis of MF and low-risk using DIPSS risk categorization OR intermediate-1 risk by DIPSS by reason of age alone.
• ET cohort: Diagnosis of ET and age ≥ 60 years OR history of thromboembolic events OR currently receiving ET-directed therapy (eg, hydroxyurea, anagrelide, interferon, busulfan, ruxolitinib, etc).
• Willing and able to provide written informed consent.
• Willing and able to complete patient assessment questionnaires either alone or with minimal assistance from a caregiver and/or trained site personnel.
• Under the supervision of a physician for the current care of MF or ET.

Exclusion Criteria:

• Individuals who are participating in blinded investigational drug studies.
• Individuals who are participating in Incyte investigational/interventional drug trials (company- or investigator-sponsored studies) until they have completed the 30-day end of study visit.
• Life expectancy ≤ 6 months.
• Diagnosis of secondary acute myeloid leukemia, myelodysplastic syndrome, chronic myelogenous leukemia, or secondary thrombocytosis.

Subjects will be enrolled from up to 250 community and academic centers across the United States.