In order to evaluate the potential usefulness of 99m technetium (Tc)-methoxy-isobutyl-isonitrile (MIBI) as a myocardial perfusion tracer in man, its myocardial distribution at rest was compared with that of 201thallium (TI). Perfusion images obtained with 99mTc-labelled microspheres, whose myocardial distribution is exclusively coronary flow dependent, were used as reference. The study was performed on twelve patients (10 males and 2 females, mean age 55.4 +/- 6.4) with suspected effort angina and without a history of previous myocardial infarction. In the space of two weeks patients underwent rest and exercise 99mTc-MIBI myocardial scintigraphy, exercise-redistribution 201TI myocardial scintigraphy, 99mTc-labelled microsphere scintigraphy after microsphere injection in the left ventricle at rest during catheterization before performing left ventriculography and coronary angiography. The comparison with microsphere distribution was limited to rest 99mTc-MIBI scintigraphy and redistribution 201TI images. Single photon emission tomography (SPECT) was employed. SPECT was performed using a double-head Rotacamera with a 360 degrees rotation arc. Ninety projections of 20 seconds each were acquired; subsequently image reconstruction was performed using an iterative algorithm. To evaluate regional perfusion the left ventricular wall was divided into 18 segments; the uptake pattern of each segment was graduated according to a qualitative score (0: severe defect; 1: moderate defect; 2: normal uptake). The quality of tomographic myocardial 99mTc-MIBI and 99mTc-microspheres images was higher than that of 201TI tomograms in account of the different physical characteristics of the two radionuclides.(ABSTRACT TRUNCATED AT 250 WORDS)