Background: Non-invasive measurement of left ventricular output has been shown to be a repeatable technique. Little is known about the repeatability using echocardiography in determining pulmonary arterial diameters or right ventricular output. Aims: To find the most repeatable point at which to measure pulmonary arterial diameter, and to compare the repeatability of determining right ventricular output with left ventricular output. Methods: We assessed the Intra-observer and inter-observer repeatability for measuring the diameter of the pulmonary trunk in 24 term and 26 preterm infants, respectively. Interobserver repeatability was assessed for the diameters of the pulmonary trunk and aorta, for stroke distance, and for left and right ventricular output. Results: The coefficients of variation for intra-observer repeatability were 4%, 7.5% and 9% respectively for measurements of the pulmonary valve, the pulmonary trunk, and the right ventricular outflow tract. There were significant differences between observers for measurement of the pulmonary trunk (p< 0.001) and right ventricular outflow tract (p= 0.011) but not for the pulmonary valve measured in either its long (p= 0.22) or short axis (p= 0.22). Significant differences between observers were also found for the pulmonary stroke distance measured in the long axis (p= 0.004) and aortic diameter at end-diastole (p< 0.001). The other parameters did not differ significantly and were used to calculate right and left ventricular output, respectively. Mean left ventricular output was 241 mls/kg/min, with mean differences between observers of 0.6 mls/kg/min (95% confidence interval (CI): −39.2 to 40.3 mls/kg/min). Mean right ventricular output was 255 mls/kg/min, with mean differences of 0.3 mls/kg/min (95% CI: −24.1 to 23.4 mls/kg/min). Conclusion: Measuring the diameter of the pulmonary trunk at the base of the valvar hinge points was most repeatable. Repeatability of right ventricular output was similar to that of left, with absolute values similar to those published by other workers.