The prospective multicenter observational cohort study will be offered consecutively to any patient with primary or secondary myelofibrosis or with Polycythemia Vera who has initiated therapy with ruxolitinib, prescribed as part of the normal course of care and completely independent of study participation. The main purpose is to assess adherence to ruxolitinib using the ARMS questionnaire. Each individual patient will be administered the questionnaire at the first convenient opportunity, regardless of when ruxolitinib is started, and again after 4, 8, 12, 24, and 48 weeks, in conjunction with drug procurement.

The prospective multicenter observational cohort study will be offered consecutively to any patient with primary or secondary myelofibrosis or with Polycythemia Vera who has initiated therapy with ruxolitinib, prescribed as part of the normal course of care and completely independent of study participation. Laboratory tests and histologic, cytogenetic, molecular, and radiologic investigations performed by the patient and collected for study will be conducted in accordance with clinical practice, independent of the patient's participation in the study. In particular, data related to systemic symptoms and splenomegaly will be collected at diagnosis and disease reassessments performed in the context of normal clinical practice. These data will be collected at the first administration of the ARMS questionnaire and again after 12, 24, and 48 weeks. If performed, any additional assessments will also be recorded. Each individual patient will be administered the questionnaire (ARMS) at the first convenient opportunity, regardless of the time of initiation of ruxolitinib, and again after 4, 8, 12, 24, and 48 weeks, in conjunction with drug procurement. If in-person data collection is not possible, the mode of data collection by telephone interview will be adopted. The minimum expected duration of individual patient observation is 48 weeks.

Inclusion Criteria:

• Age ≥ 18 years
• Patients diagnosed with Primary Myelofibrosis or secondary to Essential Thrombocythemia/Polycythemia Vera or diagnosed with Polycythemia Vera who are in the treatment with ruxolitinib therapy in accordance with normal clinical practice
• Obtaining informed consent for data collection and processing
• The Patient must come to the in-person visit at least once, an occasion on which informed consent to 'study membership will be offered

Exclusion Criteria:

• None

Patients diagnosed with primary MF or secondary to Essential Thrombocythemia/PV or diagnosed with PV, aged 18 years or older.