Magnetic resonance imaging provides an accurate method for the measurement of left and right ventricular volume. The ratio of left ventricular stroke volume to right ventricular stroke volume was calculated from contiguous transverse magnetic resonance images and was used to measure the severity of regurgitation in 18 patients with aortic regurgitation and 10 with mitral regurgitation. Cardiac anatomy was well demonstrated, allowing an assessment of relative chamber volumes and associated abnormalities, although valve abnormality was not well seen. There was a weak correlation between magnetic resonance measurements of left ventricular end diastolic volume and stroke volume ratio. The stroke volume ratio differed significantly in four groups with increasing angiographic severity of regurgitation, and all but the group with trivial regurgitation differed significantly from normal. There was good correlation between magnetic resonance and radionuclide measurements of left ventricular ejection fraction and stroke volume ratio, although the stroke volume ratio was consistently overestimated by radionuclide ventriculography. Correlation was less good for the right ventricular ejection fraction, which was underestimated by radionuclide ventriculography. It is concluded that magnetic resonance imaging provides valuable information in patients with valvar regurgitation, and serves as a suitable standard by which to judge conventional techniques.