We recently defined a high-molecular risk category (HMR) in primary myelofibrosis (PMF), based on the presence of at least one of five 'prognostically-detrimental' mutated genes (ASXL1, EZH2, SRSF2, IDH1/2). Herein, we evaluate the additional prognostic value of the 'number' of mutated genes. A total of 797 patients were recruited from Europe (n=537) and the Mayo Clinic (n=260). In the European cohort, 167 (31%) patients were HMR: 127 (23.6%) had one and 40 (7.4%) had 2 or more mutated genes. The presence of 2 or more mutations predicted the worst survival: median 2.6 years (HR 3.8, 95%CI 2.6-5.7) vs 7.0 years (HR 1.9, 95%CI 1.4-2.6) for one mutation vs 12.3 years for no mutations. The results were validated in the Mayo cohort and prognostic significance in both cohorts was independent of IPSS (HR 2.4, 95% CI 1.6-3.6) and DIPSS-plus (HR 1.9, 95% CI 1.2-3.1), respectively. Two or more mutations were also associated with shortened leukemia-free survival (HR 6.2, 95% CI 3.5-10.7), also Mayo validated. CALR mutations favorably affected survival, independently of both number of mutations and IPSS/DIPSS-plus. We conclude that the 'number' of prognostically detrimental mutations provides added value in the combined molecular and clinical prognostication of PMF.Leukemia accepted article preview online, 19 February 2014; doi:10.1038/leu.2014.76.