The results of magnetic resonance imaging (MRI) in the investigation of coarctation of the aorta were assessed and compared with those of cardiac catheterisation. This was a retrospective study of a series of 24 patients aged 14 +/- 4 years with a coarctation treated and documented by MRI. The investigation was performed with a high field 1.5 tesla (Vision, Siemens) system. Twenty-one children underwent comparative retrograde cardiac catheterisation with angiography and measurement of the peak-to-peak pressure gradient across the isthmus. No significant difference in the measurements of the aorta in MRI spin echo, gradient echo and retrograde aortic angiography were observed. On the other hand, there was a slight correlation between the degree of stenosis measured by MRI and the peak-to-peak haemodynamic gradient (r = 0.40). Seven patients had a loss of signal at the level of the aortic isthmus on MRI angiography which correlated with the haemodynamic gradient (p = 0.04). The authors conclude that MRI is a reliable non-invasive technique of investigating coarctations of the aorta. It gives accurate morphological data concerning the stenosis and blood flow. MRI should be part of the investigations of coarctation of the aorta, especially in poor indication to be able to correct it or consider the results of angioplasty or surgical correction.