Exercise training improves endothelial function in patients with chronic heart failure (CHF) through functional enhancement of circulating angiogenic cells and increased numbers of circulating endothelial progenitor cells (EPC). In contrast to healthy subjects, an immediate effect of acute exercise on CD34(+)/KDR(+) EPC is absent in CHF. Whether this reflects an attenuated or rather delayed mobilization, is addressed in the present study by measuring CD34(+)/KDR(+) EPC over a longer time period post-exercise. Seven CHF patients and eight healthy subjects (HS; 4 young and 4 age-matched subjects) underwent graded exercise testing (GXT). Venous blood was sampled before and 10, 30, and 60 min, 2, 4, 8, 12, 24 and 48 h following GXT to determine numbers of circulating CD34(+)/KDR(+) EPC (flow cytometry) and serum levels of stromal cell-derived factor (SDF)-1α (ELISA). In both HS groups, CD34(+)/KDR(+) EPC numbers increased within 10 min following GXT and remained elevated for up to 2 h. In CHF patients, the initial increase was small and normalized within 30 min. Evolution of CD34(+)/KDR(+) EPC numbers over time following GXT overall was attenuated in CHF versus HS (p = 0.036). Exercise considerably influenced SDF-1α levels over time (p = 0.0008), without a relation to the changes in CD34(+)/KDR(+) EPC. The immediate effect of acute exercise on CD34(+)/KDR(+) EPC numbers is not delayed, but significantly attenuated in CHF patients compared to HS.