Simple SummaryCytogenetic risk categorization is essential for the application of prognostic scores and the management of patients with myelofibrosis (MF). However, the low resolution of conventional karyotyping and the absence of metaphases are major limitations in MF. Herein, cytogenetic characterization via optical genome mapping (OGM) was performed in a cohort of 21 MF patients. To evaluate OGM impact on prognosis assessment, risk stratification scores were recalculated, including all the cytogenetic alterations uncovered. OGM satisfactorily characterized all patients with a previously unsuccessful karyotype. Additionally, the cryptic alterations detected via OGM led to an upgrade in the risk category for three patients. In conclusion, OGM may be a promising technique for cytogenetic assessment in MF.AbstractCytogenetic assessment in myelofibrosis is essential for risk stratification and patient management. However, an informative karyotype is unavailable in a significant proportion of patients. Optical genome mapping (OGM) is a promising technique that allows for a high-resolution assessment of chromosomal aberrations (structural variants, copy number variants, and loss of heterozygosity) in a single workflow. In this study, peripheral blood samples from a series of 21 myelofibrosis patients were analyzed via OGM. We assessed the clinical impact of the application of OGM for disease risk stratification using the DIPSS-plus, GIPSS, and MIPSS70+v2 prognostic scores compared with the standard-of-care approach. OGM, in combination with NGS, allowed for risk classification in all cases, compared to only 52% when conventional techniques were used. Cases with unsuccessful karyotypes (n= 10) using conventional techniques were fully characterized using OGM. In total, 19 additional cryptic aberrations were identified in 9 out of 21 patients (43%). No alterations were found via OGM in 4/21 patients with previously normal karyotypes. OGM upgraded the risk category for three patients with available karyotypes. This is the first study using OGM in myelofibrosis. Our data support that OGM is a valuable tool that can greatly contribute to improve disease risk stratification in myelofibrosis patients.