BACKGROUND: Evidence of a beneficial effect of exercise training on mediators of vascular disease is accumulating in chronic kidney disease (CKD), but its effect on vascular function in vivo still has to be established. The present study was designed to investigate whether a formal aerobic exercise training program improves peripheral endothelial function in patients with CKD stages 3 to 4. STUDY DESIGN: Randomized controlled trial with a parallel-group design. SETTING & PARTICIPANTS: 48 patients with CKD stages 3 to 4 without established cardiovascular disease were randomly assigned to either an exercise training program or usual care. 40 patients completed the study (exercise training, 19; usual care, 21). INTERVENTION: The 3-month home-based aerobic training program consisted of 4 daily cycling sessions of 10 minutes each at a target heart rate, calculated as 90% of the heart rate achieved at the anaerobic threshold. Patients in the usual-care group were given standard therapy. OUTCOMES: The primary outcome was peripheral endothelial function. Secondary outcomes were aerobic capacity, arterial stiffness, numbers of endothelial (EPCs) and osteogenic progenitor cells (OPCs), migratory function of circulatory angiogenic cells, and health-related quality of life. MEASUREMENTS: Endothelial function was assessed with flow-mediated dilation of the brachial artery, aerobic capacity by peak oxygen uptake (VO(2peak)), arterial stiffness by carotid-femoral pulse wave velocity, numbers of EPCs and OPCs by flow cytometry, circulatory angiogenic cell function by an in vitro migratory assay, and quality of life by the Kidney Disease Quality of Life-Short Form questionnaire. RESULTS: Exercise training significantly improved VO(2peak) and quality of life, but not in vivo vascular function (flow-mediated dilation and carotid-femoral pulse wave velocity) or cellular markers for vascular function (EPC and OPC count and circulatory angiogenic cell migratory function). LIMITATIONS: Short duration and intermittent nature of the exercise intervention. CONCLUSIONS: In patients with CKD stages 3 to 4 without overt cardiovascular disease, 3 months of aerobic exercise training improved VO(2peak) and quality of life, without altering endothelial function or arterial stiffness.