Regional myocardial utilization of glucose can be assessed non-invasively in man with the sugar analogue F18-2-fluoro-2-deoxyglucose (FDG) and positron emission tomography (PET). The preliminary observations made in patients with different clinical forms of ischaemic heart disease using FDG and PET are reported. In patients with stable angina pectoris at rest, regional myocardial glucose utilization was comparable to that in normal volunteers whilst an increased utilization of glucose was found in the recovery from exercise-induced ischaemia in the regions that showed an abnormal perfusion during the stress test. These metabolic changes persisted in the recovery phase when all the parameters that were altered during the exercise, including myocardial perfusion, had normalized. In patients with unstable angina, characterized by repeated episodes of spontaneous ST depression, myocardial glucose utilization was regionally or globally increased already at rest in the absence of symptoms or signs of ischaemia at the time of study and most often without evident perfusion abnormalities. In patients with a recent infarction, two different patterns of glucose utilization in the infarcted area have been described: a concordant decrease of myocardial perfusion and glucose utilization; a disproportionately increased glucose utilization relative to perfusion interpreted as an index of the presence of ischaemic, but still viable myocardium.