Quality of life (QoL) in patients with myelofibrosis (MF) is severely compromised by severe constitutional symptoms (i.e. fatigue, night sweats, fever, weight loss), pruritus, and symptoms from frequently massive hepatosplenomegaly. Given that no current instrument of patient reported outcomes (PRO) exists that covers the unique spectrum of symptomatology seen in MF patients, we sought to develop a new PRO instrument for MF patients for use in therapeutic clinical trials. Utilizing data from an international Internet-based survey of 458 patients with MF we created a 20-item instrument (MFSAF: Myelofibrosis Symptom Assessment Form) which measures the symptoms reported by >10% of MF patients and includes a measure of QoL. We subsequently validated the MFSAF in a prospective trial of MF patients involving patient and provider feedback, as well as comparison to other validated instruments used in cancer patients. The MFSAF results were highly correlated with other instruments, judged comprehensive and understandable by patients, and should be considered for evaluation of MF symptoms in therapeutic trials.