Exercise thallium-201 (201Tl) myocardial scans were obtained in 102 patients undergoing diagnostic coronary arteriography, of whom 59 showed more than 70% narrowing of one or more of the coronary arteries, 8 showed 50–70% narrowing of one vessel, and 35 had normal coronary arteriograms. Among the 59 patients with coronary artery disease, the exercise ECG was positive in 33 (55%) and the exercise 201Tl was abnormal in 45 (76%). 52 of these 59 patients (91%) had either an abnormal exercise 201Tl or a positive exercise ECG or both (p 201Tl correctly identified 13 cases (76%). Of 29 patients with a normal left ventriculogram, 17 (58%) showed abnormal exercise 201Tl, 28 of 30 patients (93%) with abnormal left ventriculograms showed an abnormal myocardial scan (p 201Tl compared with 22 of 34 patients (64%) without collateral circulation (p 201Tl; 4 of them also had positive exercise ECGs, and 2 showed prolapse of the mitral valve angiographically. 14 patients in this group had inconclusive exercise ECGs; and the exercise 201Tl correctly identified 13 of them (92%). We conclude that (1) exercise 201Tl improves the diagnostic value of exercise ECG; (2) the left ventriculogram and collateral circulation show a significant correlation with the incidence of abnormal exercise 201Tl; (3) false-negative exercise 201 Tl is observed predominantly in patients with disease limited to the secondary branches of the left coronary system, and (4) false-positive exercise 201Tl can be encountered in patients with prolapse of the mitral valve and with normal coronary arteriograms.