Animal experiments demonstrated that myocardial blood flow (MBF) as measured by the coincidence system and a concomitant Fick determination were very similar, varying from +9.9% to -7.9% with a mean absolute error of 5.3%. Human studies have demonstrated that (1) the average MBF in normal subjects is 269±61 ml/min/total heart, which represents 5.2±1.6% of the simultaneous cardiac output, (2) MBF determined by the coincidence technique is a reproducible measurement and has appropriate directional change with stresses imposed to augment myocardial oxygen demand, and (3) patients with significant coronary artery disease have a decreased flow reserve.