The hypoxia inducible factor 1 alpha (HIF-1 alpha) activity has been associated with various hemorrhagic events. The biological role of HIF-1 alpha in the hemorrhagic transformation of pituitary adenomas remains unknown. We hypothesized that fast growing tumor cells tend to predispose themselves to sublethal hypoxia and activate the HIF-1 alpha signaling pathway, leading to hemorrhagic transformation in pituitary adenomas. Here, we used apoplectic and non-apoplectic pituitary adenomas to determine the involvement of HIF-1 alpha signaling in intratumoral hemorrhage. We employed HIF-1 alpha overexpression/knockdown strategies to examine the association between HIF-1 alpha signaling and hemorrhagic presentation in vitro and in vivo. In support of our hypothesis, compared with non-hemorrhagic pituitary adenomas, higher cellular proliferation was observed in hemorrhagic ones and it correlated with increased HIF-1 alpha signaling. HIF-1 alpha overexpression activated its downstream genes, vascular endothelial growth factor and the proapoptotic BNIP3, in MMQ pituitary adenoma cells and this up-regulation was attenuated by HIF-1 siRNA. In vivo studies using MMQ cell xenografts in nude mice showed that HIF-1 alpha overexpression significantly promoted hemorrhagic transformation. Our study indicates that tumor hypoxia, following rapid tumor growth, may promote hemorrhagic transformation in pituitary adenomas via the HIF-1 alpha signaling pathway.