Iridium-191m would appear to be a highly useful agent for first-pass radionuclide angiocardiography (FPNA), with its very short half-life (4.96 sec), dual photopeaks (65 and 129 keV), and high injectable activity levels (greater than 100 mc). In order to compare 191mIr FPNA to current methods used to define cardiac function, 20 patients referred for cardiac catheterization were studied. Count rate data, right ventricular (RV), and left ventricular ejection fraction (LVEF), LV and diastolic volume (EDV), and end diastolic long axis (AXIS) were evaluated. Count rate data using 191mIr FPNA was consistently better than similar data obtained by 99mTc FPNA. There were acceptable correlations between 191mIr and 99mTc FPNA RVEF (r = 0.848), 191mIr FPNA and contrast angiography LVEF (r = 0.944), LVEDV (r = 0.917), and LV AXIS (r = 0.866). The data thus suggests that 191mIr FPNA has great potential in the evaluation of cardiac function.