The influence of beta blockade on the ability of ST depression, during pre-discharge exercise testing, to predict coronary anatomy and subsequent complications was studied in 300 consecutive post-infarct patients, 125 of whom underwent cardiac catheterisation. At the time of exercise 62 patients were taking a beta blocker. The exercise test had a higher sensitivity in predicting multivessel disease in patients who were not taking beta blockers than in patients who were (95% v 76%). beta Blockade did not, however, influence the ability of the test to identify patients at risk of subsequent cardiac events (sensitivity 84% and 85% respectively). These results suggest that it is not necessary to stop treatment with beta blockers before predischarge exercise testing of patients who have had an acute myocardial infarction.