To relate technetium-99m 2-methoxy-isobutyl-isonitrile (99mTc-MIBI) uptake to regional myocardial blood flow (rMBF), 99mTc-MIBI single photon emission tomography (SPET) and H2(15)O positron emission tomography (PET) scans were obtained at rest and after dipyridamole infusion in six patients with single vessel coronary artery disease. 99mTc-MIBI and H2(15)O data sets were created for each segment perfused by the stenotic vessel and for a normal reference area, assigning regions on the SPET tomograms to comparable regions on the PET by similar transaxial image reconstructions. All patients demonstrated post-dipyridamole 99mTc-MIBI perfusion defects in the territories supplied by the stenotic arteries. Resting rMBF in these regions was slightly lower than that in the normal areas (0.82 +/- 0.05 vs 0.90 +/- 0.09 ml/g/min, P = NS). A 43% +/- 14% reduction in 99mTc-MIBI activity in the area at risk was coupled with on average a 60% +/- 9% reduction in post-dipyridamole rMBF compared with control regions (0.98 +/- 0.08 vs 2.52 +/- 0.51 ml/g/min, P < 0.001). Thus, SPET assessment of 99mTc-MIBI uptake tends to underestimate the perfusion contrast between areas with normal and areas with low coronary vasodilatory reserve when compared to PET. However, these findings may still not affect the clinical usefulness of 99mTc-MIBI and more extensive studies are required to confirm these results in the clinical environment.