This study investigated performance, muscle oxygen saturation (StO2), and blood volume (BV) in patients with congenital heart diseases (CHD) and healthy children during and following sustained exercise. Maximal volunteered contraction (MVC) and endurance at 50 % of MVC (time to exhaustion, Tlim) of the knee extensor were measured in nine patients with CHD and 14 healthy control children. Near infrared spectroscopy was used to evaluated StO2 and BV in vastus lateralis. The drop in muscle oxygen saturation (D(mO2)), half time of recovery (T(SR)), and recovery speed to maximal oxygen saturation (Rs) were analyzed. Patients with CHD showed lower MVC (101.0 +/- 6.2 vs. 125.5 +/- 7.4 N x m, p < 0.01) and Tlim (67.0 +/- 7.5 vs. 127.5 +/- 11.1 s, p < 0.001) than control children. StO2 and BV values in both groups were similar at rest and decreased at the onset of contraction. D(mO2) was larger in patients, which reflected pronounced deoxygenation. During recovery, the patients exhibited a longer TSR (25.2 +/- 2.1 vs. 18.4 +/- 2.0 s, p < 0.05) and R(S) (64.6 +/- 5.5 vs. 42.7 +/- 4.6 s, p < 0.01) than control children. We concluded that reduced strength and endurance in patients with CHD were associated with an impairment of StO2 and BV, and a slower reoxygenation during recovery.