It is not clear whether there is any relationship between cardiac output and myocardial blood flow in the infarcted heart. We measured regional myocardial blood flow (RMBF) quantitatively by positron emission tomography (PET) with 13N-ammonia at rest in 18 patients with prior myocardial infarction. RMBF was calculated using the radioactivity in myocardial tissue measured by PET and the radioactivity of arterial blood. Cardiac output was determined by the dilution method using 13NH3 as an indicator, and the relation between cardiac output and RMBF was evaluated at the same time during PET study. There was a good linear correlation between cardiac index (C.I.) and mean RMBF in non-infarcted myocardium (r = 0.45, p < 0.05), and non-infarcted size (r = 0.74, p < 0.01). There was an even better linear correlation between C.I. and sigma RMBF (representing the product of mean RMBF and non-infarcted size) as follows: y = 0.016x + 1.94, r = 0.76 (p < 0.001). It was indicated that the value of C.I. may be related to RMBF of non-infarcted myocardium and its size.