Objective. A dilated aortic root is a common finding in children and adults with some forms of congenital heart defects. No data exist on root dilation in truncus arteriosus. We sought to delineate root dimensions across a population of patients with truncus arteriosus. Design. We performed a single-center retrospective review of all patients with truncus arteriosus. Demographic information, clinical history, and most recent echocardiographic data were evaluated. Results. We identified 76 patients whose most recent study was at a median age of 5.4 years (range 0-32.7 years). Mean truncal root z-score was 5.1 ± 2.3. All but three patients had truncal root z-scores greater than or equal to 2. Truncal root z-scores remained stable with increasing body surface area and age. There were no cases of dissection or rupture. Six patients underwent truncal root surgery, typically for indications of root dilation with significant truncal valve insufficiency and left ventricular dilation. Conclusions. In conclusion, mean truncal root z-score was 5, and all but three patients had truncal root z-scores greater than or equal to 2. Although repeat surgical intervention was rare and major complications related to root dilation did not occur in our cohort, further studies with longitudinal follow-up into adulthood are needed. © 2011 Copyright the Authors. Congenital Heart Disease © 2011 Wiley Periodicals, Inc.