The prognostic value of a normal exercise thallium-201 scintigram was determined in 211 patients with a normal exercise and resting scintigram. Endpoints were sudden cardiac death, non-fatal acute myocardial infarction and coronary artery bypass grafting or percutaneous transluminal coronary angioplasty. Forty patients (19%) had a history of a previous myocardial infarction and 40 (19%) were known to have had a previous percutaneous transluminal coronary angioplasty or coronary artery bypass grafting. Sixty-four patients (31%) were on treatment with beta-blocking agents. After a mean follow-up period of 23.5 months, 22 patients had had a cardiac event (1 cardiac death, 6 myocardial infarction, 15 revascularization). For the total group, the 1-year event rate for cardiac death, myocardial infarction, percutaneous transluminal coronary angioplasty or coronary artery bypass grafting was 7.0%. For cardiac death or myocardial infarction alone the event rate was 2.8%. The only parameter independently predictive for cardiac events was the regular use of beta-blocking agents. The high event rate in patients on beta-blocking treatment is partly due to the fact that these patients were more symptomatic for coronary artery disease. The sustained beta-adrenergic blockade in this patient group, even in patients advised to stop medication, was suspected to interfere with the results of 201Tl scintigraphy. Therefore, more attention should be paid to patient instruction regarding the discontinuation of medication before the test.