OBJECTIVE--To evaluate a new Doppler echocardiographic index of obstruction in patients with native coarctation or recoarctation. PATIENTS--32 patients (mean age 20, range 3 months--50 years). METHODS--Magnetic resonance imaging (MRI) was used to investigate the descending aorta. The cross sectional area of the obstruction was compared with the area of the abdominal aorta as an index of obstruction (CoA index). Doppler echocardiography was used to record the velocities in the coarctation jet and in the abdominal aorta. According to the continuity equation the ratio of these velocities should equal the ratios of the cross sectional areas. The gradients and diastolic half time of the Doppler curve were calculated. RESULTS--There was a close correlation between the MRI index of obstruction and the Doppler velocity ratio (r = 0.92). The sensitivity and specificity of this ratio in detecting a CoA index equal to or less than 0.25 were better than those obtained with gradients only or the combination of gradients and diastolic half time. CONCLUSION--The new Doppler velocity ratio correlated closely with an anatomical index of obstruction. It was easy to record in most patients and it avoided difficulties about the choice of instantaneous or peak to peak gradients and whether or not to subtract proximal velocities for the calculation of gradients.