This study was a direct comparison of the imaging characteristics of 99mTc-labeled Q12 (99mTc-Q12) and 99mTc-labeled Q3 (99mTc-Q3) in the same patients.In 10 patients with known angiographic coronary artery anatomy, 99mTc-Q12 and 99mTc-Q3 myocardial imaging were performed. Tomographic myocardial imaging was started 15 minutes after tracer injection at rest and with treadmill exercise. Ratios of heart-to-lung and heart-to-liver activity were calculated from the anterior plane projections of the tomograms at 20 minutes after tracer injection. The presence or absence of angiographic coronary artery disease was correctly determined from 99mTc-Q12 images in 10 of 10 patients with 99mTc-Q12 and in 9 of 10 patients with 99mTc-Q3. The presence or absence of a greater than 50% stenosis in an individual coronary vessel was correctly predicted for 26 of 30 vessels with 99mTc-Q12 (87%) and in 27 of 30 vessels with 99mTc-Q3 (90%, p = NS vs 99mTc-Q12). Mean heart-to-lung activity ratio at rest was 1.50 for 99mTc-Q12 and 1.93 for 99mTc-Q3 (p < 0.01). Mean heart-to-liver activity ratio at rest for 99mTc-Q12 was 0.78 versus 0.54 for 99mTc-Q3 (p < 0.0001).In 10 patients with chest pain and angiographically defined coronary anatomy, 99mTc-Q12 and 99mTc-Q3 provided similar detection of coronary artery stenoses. At 20 minutes after tracer injection, heart-to-liver activity ratios were more favorable for 99mTc-Q12 than for 99mTc-Q3, but 99mTc-Q3 resulted in a more favorable ratio of heart-to-lung activity.