To determine the accuracy of left ventricular ejection fractions (EFs) calculated from magnetic resonance (MR) images, 22 patients who underwent coronary angiography and left ventriculography were studied within 1-3 days by MR imaging. ECG-gated spin-echo 30-msec echo-delay images were obtained in end systole and end diastole in a plane through the long axis of the left ventricle perpendicular to the septum at a level through the aortic valve and apex. The area-length method was then used to calculate the EF from left ventriculograms and MR images. Angiographic EF correlated linearly with MR determinations. The mean differences in EF calculation between two observers were 5.0 +/- 6.9 for ventriculography and 2.7 +/- 7.8 for MR imaging. MR imaging, using image planes oriented to the left ventricular axes, can allow accurate EF calculation. This has the potential to be clinically useful in noninvasive evaluation of cardiac function.