As a result of deprivation of oxygen (hypoxia) and nutrients, the growth and viability of cells is reduced. Hypoxia-inducible factor (HIF)-1α helps to restore oxygen homeostasis by inducing glycolysis, erythropoiesis and angiogenesis. Here we show that hypoxia and hypoglycaemia reduce proliferation and increase apoptosis in wild-type (HIF-1α+/+) embryonic stem (ES) cells, but not in ES cells with inactivated HIF-1α genes (HIF- 1α(-/-)); however, a deficiency of HIF-1α does not affect apoptosis induced by cytokines. We find that hypoxia/hypoglycaemia-regulated genes involved in controlling the cell cycle are either HIF-1α-dependent (those encoding the proteins p53, p21, Bcl-2) or HIF1α-independent (p27, GADD153), suggesting that there are at least two different adaptive responses to being deprived of oxygen and nutrients. Loss of HIF-1α reduces hypoxia-induced expression of vascular endothelial growth factor, prevents formation of large vessels in ES-derived tumours, and impairs vascular function, resulting in hypoxic microenvironments within the tumour mass. However, growth of HIF-1α tumours was not retarded but was accelerated, owing to decreased hypoxia-induced apoptosis and increased stress-induced proliferation. As hypoxic stress contributes to many (patho)biological disorders, this new role for HIF-1α in hypoxic control of cell growth and death may be of general pathophysiological importance.