During normal lung development and in lung diseases structural cells in the lungs adapt to permit changes in lung function. Fibroblasts, myofibroblasts, smooth muscle, epithelial cells, and various progenitor cells can all undergo phenotypic modulation. In the pulmonary vasculature occlusive vascular lesions that occur in severe pulmonary arterial hypertension are multifocal, polyclonal lesions containing cells presumed to have undergone phenotypic transition resulting in altered proliferation, cell lifespan or contractility. Dynamic changes in gene expression and protein composition that underlie processes responsible for such cellular plasticity are not fully defined. Advances in molecular biology have shown that multiple classes of ribonucleic acid (RNA) collaborate to establish the set of proteins expressed in a cell. Both coding Messenger Ribonucleic acid (mRNA) and small noncoding RNAs (miRNA) act via multiple parallel signaling pathways to regulate transcription, mRNA processing, mRNA stability, translation and possibly protein lifespan. Rapid progress has been made in describing dynamic features of miRNA expression and miRNA function in some vascular tissues. However posttranscriptional gene silencing by microRNA-mediated mRNA degradation and translational blockade is not as well defined in the pulmonary vasculature. Recent progress in defining miRNAs that modulate vascular cell phenotypes is reviewed to illustrate both functional and therapeutic significance of small noncoding RNAs in pulmonary arterial hypertension.