Current prognostication in primary myelofibrosis (PMF) is based on the dynamic international prognostic scoring system (DIPSS)-plus, which employs clinical and cytogenetic variables. We recently reported DIPSS-plus independent prognostic significance for CALR (favorable) and ASXL1 (unfavorable) mutations. In the current study, 570 PMF patients were recruited for derivation (n=277) and validation (n=293) of a molecular prognostic model based on these two mutations. Survival was the longest in CALR(+)ASXL1(-) (median 10.4 years) and shortest in CALR(-)ASXL1(+) patients (median 2.3 years; HR 5.9, 95% CI 3.5-10.0). CALR(+)ASXL1(+) and CALR(-)ASXL1(-) patients had similar survival and were grouped together in an intermediate risk category (median survival 5.8 years; HR 2.5, 95% CI 1.5-4.0). The CALRsolASXL1 mutations-based prognostic model was DIPSS-plus independent (P<0.0001) and effective in identifying low/intermediate-1 risk patients with shorter (median 4 years) or longer (median 20 years) survival and high/intermediate-2 risk patients with shorter (median 2.3 years) survival. Multivariable analysis distinguished CALR(-)ASXL1(+) mutational status as the most significant risk factor for survival: HR 3.7 vs 2.8 for age >65 years vs 2.7 for unfavorable karyotype. These observations signify immediate clinical relevance and warrant i) CALR and ASXL1 mutation screening in all patients with PMF and ii) molecular revision of DIPSS-plus.Leukemia accepted article preview online, 5 February 2014; doi:10.1038/leu.2014.57.