Hypoxia-inducible factor-1 (HIF-1) is the major transcription factor specifically activated by hypoxia. It induces the expression of different genes whose products play an adaptive role for hypoxic cells and tissues. Besides these protective responses, HIF-1 and/or hypoxia have also been shown to be either anti-apoptotic or pro-apoptotic, according to the cell type and experimental conditions. More severe or prolonged hypoxia rather induces apoptosis that is, at least in part, initiated by the direct association of HIF-1α and p53 and p53-induced gene expression. On the other hand, HIF-1α dimerized with ARNT, as an active transcription factor, can protect cells from apoptosis induced by several conditions. This review is aimed to describe the different mechanisms that account for these opposite effects of HIF-1α. © 2002 Elsevier Science Inc. All rights reserved.