Pharmacologic coronary vasodilation in conjunct.ion with myocardial perfusion scintigraphy has become an alternative to dynamic exercise test for the diagnosis and risk stratification of coronary arter y disease, especially in patients who are unable to perform adequate exercise. Dipyridamole and adenosine have been used for pharmacologic stress t,esting with myocardial perfusion imaging. Adenosine is a potent, coronary vasodilator with rapid onset of action, short half Iife, near maximal coronary vasodilation and less serious side effects. ST segment depression has been reported in about 7-15% of patients with coronary artery disease receiving dipyridamole in conjunction with myocar dial perfusion imaging. The exact cause and clinical significance are not known. In order to evaluate ihe relationship between adenosine-induced ST segment depression dur- ing Tc-MIBI myocardial perfusion scin(igraphy and the severity of coronary artery disease, we performed 99mTc-MIBI imaging after int#ravenous infusion of adenosine in 120 patients with susper,t.ed coronary artery disease. Of t.he 120 patients, 28 also performed coronary angiography. There were 24 patients with ST segment depression during 99mTc-5/IIBI scintigraphy and 96 patients without ST segment depression. Adenosine was infused intravenously at. a dose of 0.14mg/kg per minute for 6minutes and 99mTc-MIB1 was injected at 3 miinute. We then compared the hemodynamic changes, side effects, scintigraphic and angiographic findings. Heart, rate increased 90 +- 19 beats/minute in t.he group with ST depression compared with 80+16 beats/rninute in the group without ST depression(p<0.05). Baseline systolic blood pressure was significantly higher in the group with ST depression(152 +- 27 mmHg) than in the group without ST depression(140 +- 21mmHg, p<0.05). Double product, at baseline(10.90 +- 2.77 versus 9.55 +- 2.34beats/minutexmmHg) and during adenosine infusion(12.72 +- 3.89 versus 10.83+2.98 beats/minutexmrnHg) were significant,ly higher in t.he group with ST de pression(p<0.05). The incidenoe of anginal chest pain was also significantly higher in the group with ST depression(75 versus 29%, p<0.0001). The 99mTc-MIBI images were abnormal in 23(96%) patients wit.h ST segment depression and 66(69%) patients without ST segment depression(p<0.05). In patients with ST segment depression, there were more reversible per- fusion defects than in patients without ST segment depression(83 versus 55%, p<0.05). The number of abnormal segments were significantly higher in the group with ST depression(3.05 +- 2.01 versus 1.51 +- 1.45, p<0.005). In patients with ST segment depression, there were more segments of reversible perfusion defects than in patients without segment depression(2.15 +- 2. 11 versus 0.89 +- 1.24, p<0.05). There were no differences in the angiographic severity by ves sel(p, NS). We concluded that ST segment depression during 99mTc-MIBI myocardial perfusion scintigraphy with intravenous adenosine is related to the severity of coronary artery disease.