In the present invention, in Pulmonary Arterial Hypertension (PAH) patient/’s blood samples (EFFORT cohort), inventors results indicate that activin–Smad2/3 signaling is overactive, as reflected by the overabundance of inhibin-βA, inhibin-βB, activin type-I and type-II receptors, and phospho-Smad2/3 in PAH vascular endothelial and smooth muscle cells and by the fact that elevated levels of activin A and follistatin-like 3 (FSTL3) level in serum predict adverse outcome. With an independent external PAH cohort, inventors confirmed that both activin A and FSTL3 are prognostic biomarkers in PAH. Thereafter, this approach allows to identify a BMP/TGF ligands combinations that represent a reliable biomarker of PAH severity and/or mortality, validated in a second independent cohorts of PAH patient (Imperial College of London, UK study) 2-biomarker panel composed of activin A and follistatin-like 3 (FSTL3) that was independently associated with prognosis both at the time of PAH diagnosis and at the first follow-up after PAH therapy initiation. More specifically present invention relates to methods for prognosis and/or monitoring of the severe form of Pulmonary Hypertension (PH) and PAH through comparison of specific markers combinations in PH or PAH patient.