Patients who carried primary or secondary myelofibrosis from Philadelphia negative MPNs (PMF/SMF) and who are treated or are about to be treated with pegylated-interferon (mostly α2a) are eligible to this prospective study. Biological and clinical parameters will be collected from the beginning of the drug use until last news.

A non-opposition consent form need to be signed before entering this study.

In all centres interested, patients who were diagnosed for PMF/SMF can be treated with oral or sub-cutaneous drugs in order to reduce clinical symptoms and biological abnormalities belonging to these diseases.

Pegylated-interferon is one of the better weapons that can be used in these cases. These treatments are well known and used from many years in french hematological centres.

The investigators to collect clinical and biological data from patients treated with pegylated-interferon α2a (the most use) in case of PMF/SMF in order to assess efficacity and tolerance to this drug. The prescription of the product and the dose are under the responsibility of each practician.

Clinical data collected: disease and patients characteristics at the time of diagnosis and at the beginning of the drug use, spleen and liver size, presence of constitutive symptoms, transfusion needs.

Biological data collected: complete hemogram, CD34+ cells count, allele burden of JAK2V617F and calreticulin clones if DNA samples are collected.

These data will be collected avery 3 months during the two first years and every 6 months after.

The investigators also recorded adverse side effects if significative, the concomitant use of hematological drugs (cytoreductive treatments, ASE...), modalities of prescription of the drugs and the reason for stopping the drug and etiology of death if happened.

Inclusion Criteria:

• Primary or secondary Myelofibrosis
• Prescription of pegylated interferon α2a
• Age > 18 years-old

Exclusion Criteria:

• Other MPNs treated with pegylated interferon α2a
• Patients treated for PMF or SMF but without pegylated interferon

Patients diagnosed for PMF or SMF and treated with pegykated interferon α2a