Background The objective was to better understand Doppler hemodynamics and exercise capacity in patients with Fontan palliation by delineating the hemodynamic mechanism for temporal changes in their peak oxygen consumption (V̇O2). Methods We performed a retrospective review of adult Fontan patients with systemic left ventricle (LV) who underwent serial transthoracic echocardiograms (TTE) and cardiopulmonary exercise tests (CPET) at Mayo Clinic in 2000-2015. TTE and CPET data were used (1) to determine agreement between V̇O2 and Doppler-derived LV function indices (eg, stroke volume index [SVI] and cardiac index [CI]) and (2) to determine agreement between temporal changes in peak V̇O2 and LV function indices. Results Seventy-five patients (44 men; 59%) underwent 191 pairs of TTE and CPET. At baseline, mean age was 24 ± 3 years, peak V̇O2 was 22.9 ± 4.1 mL/kg/min (63 ± 11 percent predicted), SVI was 43 ± 15 mL/m2, and CI was 2.9 ± 0.9 L/min/m2. Peak V̇O2 correlated with SVI (r = 0.30, P <.001) and with CI (r = 0.45, P <.001) in the 153 pairs of TTE and CPET in patients without cirrhosis. Temporal changes in percent predicted peak V̇O2 correlated with changes in SVI (r = 0.48, P =.005) and CI (r = 0.49, P =.004) among the 33 patients without interventions during the study. In the 19 patients with Fontan conversion, percent predicted peak V̇O2 and chronotropic index improved. Conclusions Overall, there was a temporal decline in peak V̇O2 that correlated with decline in Doppler SVI. In the patients who had Fontan conversion operation, there was a temporal improvement in peak V̇O2 that correlated with improvement in chronotropic index.