We have shown that statistically significantly higher percentages of CD4+CTLA-4+ and CD8+ CTLA-4+ T cells, measured at the time of iPAH diagnosis, were found in patients who died within 5 years of the diagnosis, which allows us to consider both of the above lymphocyte subpopulations as a negative prognostic/predictive factor in iPAH. This evidence concerns the gene CD4 and idiopathic pulmonary arterial hypertension.