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In order to assess the changes in the number and severity of transient and persistent myocardial perfusion defects before and after coronary artery bypass grafting (CABG), early and late post-exercise thallium-201 (tl-201) tomography was performed in 25 patients, on average 10 days before and 2.5 months after CABG. Six patients had a previous Q wave myocardial infarction. Comparing the pre- and post-CABG exercise test data (symptom limited bicycle ergometry) showed an improvement in working capacity (from 129±34 to 145 ±35 W, P<0.05) and a reduction in exercise induced angina from 77 to 9% of the patients(P<0.001). From the tomographic data, six short axis and three sagittal cross-sections of the left ventricular myocardium were reconstructed. In each patient study, a total of 51 myocardial segments were defined and analysed in a semi-quantitative manner by means of a five point scoring method indicative of the severity of a defect (from 0 = normal tl-201 uptake to 4 = absence of tl-201 activity). In this patient population the total number of segements with transient tl-201 perfusion defects decreased from 11.0±7 segments per patient (mean±SD; before surgery to 4.9±4 after surgery (p<0.01); the number of segments with ‘persistent’ tl-201 defects decreased as well from 7.5±6 to 3.9±4 segments per patient (P<0.03). On average, the severity score of transient perfusion defects decreased from 17.7±12 to 7.1±6 per patient (mean±SD.P<0.01) and that of ‘ persistent ’ defects from 17.4±15 to 8.4 ± 9 (p<0.03). From these data it may be concluded, that the apparent reversibility of certain tl-201 defects are compatible not only with scar tissue, but also with a reversibly ischaemic myocardium. [ABSTRACT FROM PUBLISHER]