In addition to preoperative coronary angiography, 63 patients undergoing coronary bypass surgery also had myocardial scintigraphy with dipyridamole 1 to 2 days before surgery, resting scintigraphy 10 after bypass and a repeat scintigraphy with dipyridamole 3 to 6 months later. The myocardium was divided into 5 segments. In each segment the perfusion was classified as: normal, presence of a reversible defect (redistribution), presence of a permanent defect (no redistribution). A comparison of pre-and postoperative scintigraphies led to the following conclusions. Three to six months after bypass, improved perfusion was observed in 65.6% os ischaemic segments, showing that surgery was very effective. On the other hand, the immediate postoperative control (10 days) only showed improved perfusion in 42.5% of the ischaemic segments: this interval was too short to appreciate the benefits of coronary bypass. There was a good correlation between the scintigraphic improvement after surgery and the reversibility of the zones of hypofixation of Thallium before surgery. When the defect was reversible, 80% of the revascularised segments were improved. Fifty-eight per cent of non-reversible defects before surgery were unchanged by the revascularisation procedure. However, the absence of redistribution during preoperative scintigraphy was not synonymous with definitive myocardial lesions and does not represent a contra-indication to coronary bypass surgery; in fact, improved perfusion was observed in 42% of these constant defects. The quality of recovery depends on the condition of the muscle: only segments with normal motion can be improved. Degradation of the clinical condition of patients was always related to aggravation(lack of improvement in 1 case) of scintigraphy.(ABSTRACT TRUNCATED AT 250 WORDS)