A method which does not require an online computer has been developed to measure left ventircular ejection fraction from a bolus of 99mTc-pertechnetate. The passage of the bolus through the cardiac chambers was imaged using a gamma camera and recorded on video tape. On replay a region of interest corresponding to the left ventricle was selected directly at the gamma camera console. Counts detected in this region were analysed in 30ms intervals. The data were corrected for the contribution from extra-cardiac activity and were then digitally filtered to remove statistical noise. Ejection fractions measured in 13 patients by this technique agreed well with those determined by contrast ventriculography (r=0.97, P less than 0.001).