Because of numerous reports of false positive results with thallium-201 (Tl-201) stress testing in patients with left bundle branch block, the authors decided to evaluate another mode of coronary vasodilatation, dipyridamole, for the diagnosis of coronary atheromatosis. Nine patients were prospectively studied with stress and dipyridamole Tl-201 scintigraphy; both tests were performed within three to 79 days of one another. Five of the patients also had coronary angiograms (four within one year, one five years earlier). Four of the patients had normal results with both tests (two normal angiograms, two not performed); two had reversible septal defects with stress-induced coronary vasodilatation but normal dipyridamole studies (only one had an angiogram, which was normal); one patient had a fully reversible septal defect with stress and a fixed defect with dipyridamole (normal angiogram); one had a partially reversible septal stress defect which was fixed with dipyridamole; and one had a normal stress study but a reversible septal defect with dipyridamole (an angiogram performed five years earlier showed 30 to 40% stenosis of the anterior descending artery). Because it seems that dipyridamole produces fewer false positive results, it should be used instead of stress testing to induce coronary vasodilatation in patients with left bundle branch block.