40 patients underwent aortocoronary bypass surgery for coronary artery disease involving a single vessel in 2 cases, 2 vessels in 10 cases, and all 3 vessels in 28 cases. Preoperative exercise Thallium 201 scanning showed one or several zones of hypofixation (80 p. 100 of maximum myocardial fixation) which were not present at rest. After surgery, exercise scanning was normal in 22 out of 24 patients with normalised or improved vascularisation on control angiography, but remained pathological in 2 cases. However, comparable areas of hypoperfusion were observed in 16 patients with no improvement or worsening of vascularisation after operation. The exercise EG remained pathological in 6 out of 24 patients with normalised or improved vascularisation and in 11 out of 16 patients with identical or worsened vascularisation. In addition, 10 of the 24 patients with improved or normalised vascularisation, and 12 of 16 patients with identical or worsened vascularisation, continued to have chest pain after operation. A zone of hypoactivity was observed in the preoperative exercise scan in 18 out of 20 patients with isolated stenosis treated by catheter dilatation. Normalised exercise scans were recorded in all patients in the month following dilatation, but the exercise ECG remained pathological in 3 patients. In conclusion, 201 Thallium scanning gives a good assessment of myocardial vascularisation after revascularisation procedures. It helps identify patients with non-anginal post-operative chest pain, and detects residual ischaemia in patients without ischaemic signs on electrocardiography.