Introduction: Circulating progenitor cells (CPC) are a heterogeneous population of stem/progenitor cells in peripheral blood that participate in tissue repair. CPC mobilization has been well characterized in able-bodied persons but has not been previously investigated in wheelchair racing athletes. The purpose of this study was to characterize CPC and CPC subpopulation mobilization in elite wheelchair racing athletes in response to acute, upper-extremity aerobic exercise to determine whether CPC responses are similar to ambulatory populations. Methods: Eight participants (three females; age = 27.5 T 4.0 yr, supine height = 162.5 T 18.6 cm, weight = 53.5 T 10.9 kg, V? O2peak = 2.4 T 0.62 LIminj1, years postinjury = 21.5 T 6.2 yr) completed a 25-km time trial on a road course. Blood sampling occurred before and immediately after exercise for quantification of CPC (CD34+), hematopoietic stem and progenitor cells (HSPC) (CD34+/CD45dim), hematopoietic stem cells (HSC) (CD34+/CD45dim/CD38j), CD34+ adipose tissue (AT)-derived mesenchymal stromal cells (MSC) (CD45j/CD34+/CD105+/CD31j), CD34j bone marrow (BM)-derived MSC (CD45j/CD34j/CD105+/CD31j), and endothelial progenitor cells (EPC) (CD45dim/CD34+/VEGFR2+) via flow cytometry. Blood lactate was measured before and after trial as an indicator of exercise intensity. Results: CPC concentration increased 5.7-fold postexercise (P = 0.10). HSPC, HSC, EPC, and both MSC populations were not increased postexercise. Baseline HSPC populations were significantly positively correlated to absoluteV? O2peak (rho = 0.71, P G 0.05) with HSC trending to positively correlate to V? O2peak (rho = 0.62, P = 0.10). AT-MSC populations were trending to be negatively correlated to baseline V? O2peak (rho = j0.62, P = 0.058). The change in CPC, EPC, and AT-MSC pre- and postexercise significantly positively correlated to the change in lactate concentrations (rho = 0.91 P = 0.002, 0.71 P = 0.047, 0.81 P = 0.02, respectively, all P G 0.05). Conclusion: These data suggest that CPC content in wheelchair racing athletes is related to cardiorespiratory fitness, and responses to exercise are positively related to exercise intensity.