OBJECTIVES: Conducting real world evidence (RWE) research on patients with rare diseases is particularly challenging when using only one type of data (e.g. insurance claims). Myelofibrosis (MF) is a rare, hematological cancer with global annual incidence rate of <1 - 2.4 cases per 100,000 patients. MF is characterized by the reduced ability of bone marrow to synthesize blood cells which can result in anemia, thrombocytopenia, and higher risk of infection. Absent adequate, recent data on MF patients, a practice-based network (PBN) data platform was developed to facilitate RWE research on MF patients. METHODS: MF patients observed in IMS Health databases between Nov-2010 to Oct-2012 were identified. Academic centers of excellence and large community oncology practices treating MF patients were then identified to link additional patients into the platform using a HIPAA-compliant patient de-identification algorithm. Patient demographics and attributes of dispensed prescriptions, private practitioner visits, and electronic medical record data including laboratory information were collected across patients from Jan-2000 to Mar-2013. Descriptive analyses of demographic and clinical characteristics were conducted to assess the generalizability of the sample vs. literature. RESULTS: 6,362 U.S. MF patients were identified in the platform. Mean (SD) age was 67 (12.6) and 45% of patients were female. Mean (SD) Charlson Comorbidity Index was 4.36 (2.4). Among the subset of 529 patients with laboratory results, 63% were identified as anemic and 34% had platelet counts 50,000 - 100,000 / microL. All U.S. geographic regions were represented. CONCLUSIONS: Demographic and clinical results suggest that this large sample of MF patients is comparable to prior estimates of the broader MF population. This case study of U.S. MF patients suggests it is important to look beyond any one data source and to build PBN platforms with key clinical domains spanning multiple geographic regions when conducting RWE research on patients with rare diseases.