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Tl myocardial imaging was performed at rest and after dipyridamole (0.44 mg/kg) on 50 patients with known or suspected ischemic heart disease. The dipyridamole had no effect in 14 patients (group Dip. 0). In 17 patients (group Dip±) it significantly modified the contrast of the rest image (by increasing or decreasing a rest perfusion defect). In 19 patients (group Dip-Steal) the drug induced a paradoxical response interpreted as a coronary steal effect (an active region at rest becomes hypoactive after dipyridamole while an underperfused region at rest improves). All patients underwent coronary arteriography and left monoplane ventriculography; results were interpreted in relation to these angiographic data. The mean percentage of stenoses (per patient) was about the same in the three groups but it was found that, despite these stenoses, the patients of the group Dip -Steal had a good left ventricular function ( $${{\text{EF}}}$$ =0.62±0.12). On the other hand, the ejection fraction was very poor in the two other groups (0.50±0.17 and 0.48±0.17). Moreover it was found that: (1) the frequency of high grade or even complete obstruction was notably less in group Dip-Steal ( P<0.05); (2) the frequency of angiographically visible collaterals was higher in group Dip-Steal ( P<0.05); (3) the left anterior descending artery was less diseased than the right coronary artery in group Dip-Steal ( P<0.05). These results have a real prognostic value for the assessment of the preserved cardiac performance in Dip-Steal patients despite severe stenoses, and are discussed in terms of compensatory collateral circulation and preservation of the coronary-flow reserve in the myocardium distal to a critical stenosis. [ABSTRACT FROM AUTHOR]