Hypoxia-inducible transcription factors (HIFs) regulate a wide array of adaptive responses to hypoxia and are often activated in solid tumors and hematological malignancies due to intratumoral hypoxia and emerging new layers of regulation. We found that in chronic lymphocytic leukemia (CLL) HIF-1α is a novel regulator of the interaction of CLL cells with protective leukemia microenvironments, and is in turn regulated by this interaction in a positive feedback loop that promotes leukemia survival and propagation. Through unbiased microarray analysis we found that in CLL cells HIF-1α regulates the expression of important chemokine receptors and cell adhesion molecules that control the interaction of leukemic cells with bone marrow and spleen microenvironments. Inactivation of HIF-1α impairs cell adhesion to stroma and chemotaxis, reduces bone marrow and spleen colonization in xenograft and allograft CLL mouse models, and prolongs mice survival. Interestingly, we found that in CLL cells HIF-1α is transcriptionally regulated upon coculture with stromal cells. Furthermore, HIF-1α mRNA levels vary significantly within CLL patients, and correlate with the expression of HIF-1α-target genes including CXCR4, thus further emphasizing the relevance of HIF-1α expression to CLL pathogenesis.