p21^Cip1 (p21) controls cell cycle progression and apoptosis in mature endothelial cells (ECs) and regulates size and cycling of the hematopoietic progenitor cell pool. Because circulating endothelial progenitor cells (EPCs) contribute to postnatal neovascularization in addition to mature ECs, we investigated the regulation of ECs and EPCs in p21-deficient mice. Mature aortic EC proliferation was increased in homozygous p21^−/− and heterozygous p21^+/− mice, in which p21 protein levels are reduced to one third of wild-type (WT). In contrast, apoptosis sensitivity was increased by 3.5-fold only in p21^−/−, but not in p21^+/− mice. Consistently, in vivo apoptosis of ECs within areas of neovascularization was elevated in p21^−/− but not in p21^+/− mice. EPC numbers were elevated 2-fold in p21^−/− mice compared with WT (P<0.001), and clonal expansion capacity of EPCs was increased from 25±4 (WT) to 57±8 colony-forming units in p21^−/− mice (P<0.005). EPC numbers and expansion were likewise increased in p21^+/− mice. As the integrative endpoint, in vivo neovascularization reflecting all p21-affected parameters was increased over WT only in p21^+/− (P<0.001), but not in p21^−/− mice. In conclusion, reduced p21 protein levels of mice lacking one p21 allele are associated with increased proliferation of ECs and EPCs, whereas survival of ECs to apoptotic stimuli in vitro and in vivo is not impaired. Under these conditions, neovascularization was increased. In contrast, complete p21 deficiency did not result in an increased neovascularization despite increased mature EC and EPC proliferation. This may be due to the sensitization of ECs against apoptosis.