We found that FIZZ1/RELMα is inducible by hypoxia in lung. The hypoxia-upregulated expression of FIZZ1/RELMα was located in the pulmonary vasculature, bronchial epithelial cells, and type II pneumocytes. Recombinant FIZZ1/RELMα protein stimulates rat pulmonary microvascular smooth muscle cell (RPSM) proliferation dose-dependently. Therefore, we renamed this gene as hypoxia-induced mitogenic factor (HIMF). HIMF strongly activated Akt phosphorylation. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 inhibits HIMF-activated Akt phosphorylation. It also inhibits HIMF-stimulated RPSM proliferation. Thus, the PI3K/Akt pathway, at least in part, mediates the proliferative effect of HIMF. HIMF also has angiogenic and vasoconstrictive activity. Notably, HIMF increases pulmonary arterial pressure and vascular resistance more potently than either endothelin-1 or angiotensin II.