Abstract: Functions of the adventitia were often neglected in previous studies of hypoxic pulmonary vascular remodeling. Recent studies show that vascular adventitia, especially its fibroblasts, plays an important role in vascular function. Adventitia fibroblasts are the primary sensory cells under hypoxic condition and hypoxia can directly activate adventitia fibroblasts. Activated fibroblasts not only synthesize and secrete growth factors and cytokines to promote the proliferation of smooth muscle cells, but also transdifferentiate into smooth musclelike cells (myofibroblasts) and secrete collagen, resulting in vascular remodeling. Therefore, hypoxiainduced fibroblast activation plays a vital role in hypoxic pulmonary vascular remodeling and hypoxic pulmonary hypertension.