The effects of exercise testing 3 weeks after clinically uncomplicated myocardial infarction (MI) on subsequent physical activity were evaluated in 40 consecutive men with a mean age of 52 +/- 9 years. Patients' confidence in their ability to perform various physical activities was evaluated with self-efficacy scales which patients completed before and after a symptom-limited treadmill exercise test. Increases in confidence (self-efficacy) for activities similar to treadmill exercise (walking, stair climbing, and running) were greatest after treadmill exercise, whereas increases for dissimilar activities (sexual intercourse and lifting) were greatest after test results were explained by a physician and nurse. The intensity and duration of subsequent physical activity at home were more highly correlated with self-efficacy after treadmill exercise than with peak treadmill heart rate. Of the 8 patients whose treadmill tests were limited by angina pectoris, 7 had self-efficacy scores which remained low after treadmill testing or which decreased from initially high values after treadmill testing. These patients had lower peak heart rates and work loads than patients whose self-efficacy increased or remained high after treadmill testing. After MI, patients' perception of their capacity for physical activity and their actual patterns of subsequent physical activity are influenced by early treadmill testing in a manner which is congruent with these patients' treadmill performance.