The long-term outcome after repair of tetralogy of Fallot (TOF) is critically dependent pulmonary valve competence that is compromised by trans-annular patch (TAP). We compared a new echocardiographic index [pulmonary annulus index (PAI)] to conventional methods of predicting need for TAP in infants undergoing TOF repair. Consecutive infants undergoing TOF repair were prospectively studied. Pre-operative aortic and pulmonary annuli and main pulmonary artery (MPA) diameters were measured and z scores determined. PAI was a ratio of observed to expected pulmonary annulus (PA) diameter. TAP was based on intra-operative sizing by surgeons blinded to PAI values. Receiver operator curves (ROC) were generated for all PAI, MPA z scores and pulmonary annulus z scores. Of 84 infants (8.6 ± 2.6 months; 7.5 ± 1.3 kg), 36 needed TAP (43%). All the three indices viz. PAI, Pulmonary annulus and MPA z scores performed similarly in predicting need for TAP (ROC curves ~ 80%). Combining cut-offs of MPA z scores (> − 3.83) with either PAI (> 0.73) or PA z score (> − 1.83) predicted avoidance of TAP with ~ 90% accuracy. When both PAI and MPA z scores were below the cut-offs there was an 80% likelihood of TAP. Failure to predict TAP was associated with unicommisural pulmonary valves. PAI was equal to PA z scores in predicting need for TAP during repair of TOF. Combining either with MPA z scores was the most accurate method of prediction. Failure of prediction was mainly due to presence of a unicommissural pulmonary valve.