Background: Performance of the functionally single right ventricle may deteriorate over time. Quantitative assessment of this chamber, however, is complicated by its asymmetric geometry. Automatic detection of borders, and the Doppler-derived index of myocardial performance, are echocardiographic techniques that allow for quantitative assessment regardless of ventricular shape. We sought to evaluate the mechanics of contraction and relaxation in the functionally single right ventricle using these parameters. Methods: We evaluated systemic ventricular function in 35 asymptomatic patients with functionally single right ventricle, having a mean age of 7.8 ± 3.1 years, who had undergone the Fontan procedure. We compared them with 32 age-matched normal controls using both automatic detection of borders and Doppler indexes. Results: When compared with the controls, the group with a functionally single right ventricle demonstrated diminished systolic function as evidenced by a lower fractional change in area (42.7 ± 10.1% vs. 54.6 ± 10.5%, p = 0.001), and diminished diastolic function, as demonstrated by a greater reliance on atrial contraction to achieve ventricular filling (32.0 ± 4.4% vs. 22.2 ± 4.1%, p = 0.001). The mean index of myocardial performance in those with functionally single right ventricles was also greater than in controls (0.41 ± 0.12 vs. 0.30 ± 0.05, p = 0.001), and the indexed ejection time was shorter (0.35 ± 0.05 vs. 0.39 ± 0.05, p = 0.01), suggesting less efficient ventricular mechanics. Conclusions: These data demonstrate that the systolic and diastolic properties of the functionally single right ventricle differ significantly from those of the normal systemic left ventricle. Use of the echocardiographic techniques provide insight into ventricular mechanics in patients with functionally single ventricles, and may be valuable tools for serial quantitative follow-up.