BACKGROUND Congenital bicuspid aortic valve (BAV) is the most common congenital heart defect and may be responsible for aortic stenosis early in life. However, its pathogenesis remains unclear. A relationship between the severity of aortic stenosis and valvular surfaces has not been reported in the paediatric population. OBJECTIVE To assess the feasibility of three-dimensional transthoracic echocardiographic planimetry in congenital BAV in children and to evaluate the influence of valvular asymmetry and aortic valve area (AVA) on stenosis severity. METHODS Seventy consecutive children with BAV were included in this prospective single-centre study. Using the multiplanar review mode, surfaces were measured by planimetry (in systole for AVA and diastole for cusp surfaces). The degree of stenosis was assessed by instantaneous aortic Doppler. Results are expressed as medians and first and third quartiles. RESULTS Median age was 5.6 years (2.2-11.5). Feasibility was 97%. Intra- and interobserver concordances were excellent for the measurement of cusp surfaces and AVA. Among the 70 children, 25 had aortic stenosis. The small/large cusp ratio was strongly associated with aortic stenosis (P<0.001). The area under the receiver operating characteristic curve was 0.89 (95% confidence interval 0.82-0.97). The best cut-off value for differentiating stenotic from non-stenotic valve was 0.75, with 84% sensitivity and 83% specificity. When indexed for body surface area, AVA was significantly smaller (P=0.031) in case of stenotic BAV (1.51cm(2) [0.99-2.28]) compared with non-stenotic BAV (1.99cm(2) [1.57-2.52]). CONCLUSIONS Three-dimensional echocardiographic planimetry is a feasible and reproducible method for assessing aortic surfaces in congenital BAV. Aortic stenosis seems to strongly depend on the asymmetry of the valve.