OBJECTIVE--To test the hypothesis that isolated coarctation of the aorta is associated with relative hypoplasia of the mitral valve, even when the valve is morphologically normal. DESIGN--Cross sectional and Doppler echocardiography were used in a prospective, paired, case control study to compare mitral valve dimensions and diastolic transmitral flow characteristics as indices of left heart development. 40 children with isolated coarctation and 40 size matched controls were examined. Within the coarctation group 14 children with apical diastolic murmurs were compared with 14 size matched patients without murmurs. SETTING--A supraregional tertiary referral centre for paediatric cardiology. OUTCOME MEASURES--Mitral valve diameters, measured from the parasternal long axis, short axis, and apical four chamber views; mitral valve cross sectional area measured from the parasternal short axis view; peak early (E) and peak atrial (A) phase diastolic transmitral flow velocities measured by pulsed wave Doppler from the apical four chamber view; derived E/A ratio and pressure half time of decay from peak E. RESULTS--Mitral valve dimensions were significantly smaller in children with coarctation than in controls for long axis diameter (median 1.74 v 1.90cm, p = 0.0001), short axis diameter (2.21 v 2.28 cm, p = 0.027), and cross sectional area (2.37 v 3.15 cm2, p = 0.001). Peak E and A velocities were significantly higher in patients than in controls (0.9 v 0.82 ms-1, p = 0.013 and 0.61 v 0.51 ms-1, p = 0.007). The only difference between children with coarctation plus murmurs and those without murmurs was a marginally longer pressure half time. CONCLUSIONS--Smaller mitral valve dimensions and increased diastolic transmitral flow velocities in children with isolated coarctation compared with normal children suggests that coarctation may be part of a generalised hypoplasia of left heart structures.