In Mustard/Senning-repaired (MSR) patients, the right and left ventricles (RV, LV) act as the systemic and pulmonary ventricle, respectively. The purpose of the study was to compare non-invasively, at rest, pulmonary ventricle systolic function and hemodynamics in MSR patients with those of healthy volunteers. Velocity-encoded MR imaging was performed at the level of the main pulmonary artery (MPA) in ten male patients late after a Mustard/Senning correction performed early in infancy and in ten male volunteers. Both blood flow and MPA cross-sectional area variations were recorded over a complete cardiac cycle. MPA distensibility, body surface area (BSA)-normalized pulmonary ventricle systolic power and work were significantly lower in the MSR patients compared to volunteers. In particular, BSA-normalized LV systolic power and work in MSR patients were equal to 82 and 77% on average of those of the RV in volunteers (0.32 vs. 0.39 W/m2 and 0.10 vs. 0.13 J/m2), respectively. We conclude that in MSR patients at rest two unrelated findings were observed: (1) a reduced MPA distensibility and (2) a significantly lower systolic mechanical performance of the pulmonary LV compared to that of the RV in healthy volunteers. The latter quantification indirectly confirms the lowest systemic RV systolic mechanical performance previously published.