Because of its quantitative power and capacity to image biochemically unaltered tracers of physiological substrates, cardiac positron-emission tomography has emerged as a promising diagnostic modality and a powerful clinical research tool. It has enhanced characterization of the efficacy of diverse interventions designed to improve myocardial perfusion, differentiation of reversibly from irreversibly injured ischaemic myocardium, and detection of cardiomyopathic states. Progress in high rates of data acquisition, improved spatial and temporal resolution, accelerated image processing, and 3-dimensional display of images of myocardium offer promise for widespread applications to early detection of cardiac allograft rejection, graft atherosclerosis, and the efficacy of pharmacological interventions designed to protect recipients of cardiac allografts.