OBJECTIVETo evaluate whether counts of circulating colony forming unit-endothelial cells (CFU-ECs), cells co-expressing CD34, CD133, and CD31 (CD34+CD133+CD31+), and CD34+CD45- cells are altered in adolescents with type 1 diabetes and if the changes in counts correlate with endothelial dysfunction.STUDY DESIGNAdolescents with diabetes (ages 18 to 22 years) and race- and sex-matched control subjects were studied. We assessed circulating CFU-ECs, using colony assays, and CD34+CD133+CD31+ and CD34+CD45- cells, using poly-chromatic flow cytometry. CFU-ECs and CD34+CD133+CD31+ are hematopoietic-derived progenitors that inversely correlate with cardiovascular risk in adults. CD34+CD45- cells are enriched for endothelial cells with robust vasculogenic potential. Vascular reactivity was tested by laser Doppler iontophoresis.RESULTSSubjects with diabetes had lower CD34+CD133+CD31+ cells, a trend toward reduced CFU-ECs, and increased CD34+CD45- cells compared with control subjects. Endothelium-dependent vasodilation was impaired in subjects with diabetes, which correlated with reductions in circulating CD34+CD133+CD31+ cells.CONCLUSIONSLong-term sequelae of type 1 diabetes include vasculopathies. Endothelial progenitor cells promote vascular health by facilitating endothelial integrity and function. Lower CD34+CD133+CD31+ cells may be a harbinger of future macrovascular disease risk. Higher circulating CD34+CD45- cells may reflect ongoing endothelial damage. These cells are potential biomarkers to guide therapeutic interventions to enhance endothelial function and to prevent progression to overt vascular disease.