Recent experiments show that hematopoietic progenitor cell populations contain endothelial precursor cells. We have isolated a population of CD34 + cells that expresses fibroblast growth factor receptor-1 (FGFR-1) and that differentiates into endothelial cells in vitro. We find that 4.5% ± 2.1% of CD34 + cells isolated from bone marrow, cord blood, and mobilized peripheral blood express FGFR-1 and that viable CD34 +FGFR + cells are small, with little granularity, and express both primitive hematopoietic and endothelial markers on their surface. The primitive hematopoietic markers AC133, c-kit, and Thy-1 are coexpressed by 75%, 85%, and 64% of CD34 +FGFR + cells, respectively. Most of the CD34 +FGFR + cells also express antigens found on endothelial cells, such as CD31, vascular endothelial growth factor receptor-2, and the endothelial-specific cell surface marker, vascular endothelial cadherin (VE-cadherin), whereas 56% to 60% of the cells express Tie, Tek, and the endothelial-specific marker, P1H12. The CD34 +FGFR + population is enriched in cells expressing endothelial-specific antigens compared with the CD34 + population. Isolated CD34 +FGFR + cells grow slowly in culture, are stimulated by fibroblast growth factor-2 and vascular endothelial growth factor, and give rise to cells that express von Willebrand factor and VE-cadherin and that incorporate acetylated low-density lipoprotein. These experiments show that FGFR-1 is expressed by a subpopulation of CD34 + cells that give rise to endothelial cells in vitro, indicating that this population contains endothelial stem/progenitor cells.