On exercise testing after an episode of unstable coronary artery disease (CAD; unstable angina or non-Q-wave myocardial infarction), a proportion of patients show ST-segment depression, indicating myocardial ischaemia, but do not report concomitant symptoms of angina. Treatment of such "silent" ischaemia aims mainly to reduce the risk of subsequent cardiac events. We have studied the effect of low-dose aspirin in patients with myocardial ischaemia defined at the predischarge test as silent (though patients might have had symptomatic ischaemia at other times) or symptomatic. 740 men with unstable CAD aged 70 years or less underwent symptom-limited exercise testing before hospital discharge; 144 showed ST depression without pain and 230 ST depression with simultaneous chest pain. Of the silent ischaemia group, 67 were randomly assigned placebo and 77 aspirin (75 mg daily); the corresponding numbers in the symptomatic group were 125 and 105. Angina symptoms were less common in the silent than in the symptomatic ischaemia group both before inclusion and during follow-up, and a greater proportion of the silent ischaemia group were included because of myocardial infarction. In both ischaemia groups aspirin treatment reduced the risk of subsequent myocardial infarction or death by 3 months' follow-up (silent 4% of aspirin-treated vs 21% of placebo-treated patients, p = 0.004; symptomatic 9% vs 18%, p = 0.05); at 12 months' follow-up a significant benefit of aspirin was still apparent in the silent ischaemia group (9% vs 28%, p = 0.005) but not in the symptomatic group (13% vs 22%, p = 0.109). Low-dose aspirin reduced the risk of subsequent myocardial infarction at least as well in silent as in symptomatic myocardial ischaemia. Since improvement of outlook is the main treatment objective in symptom-free patients, aspirin should be a mainstay of their treatment.