Episodes of transient myocardial ischemia during daily life were investigated in 30 patients on two separate occasions, by ambulatory Holter ST monitoring. The first occasion was at a time of uncertainty in the patients' lives, when the results of coronary angiography and the need for surgery were to be discussed. The second was at a later date, when there had been time to adjust to the decision-making process. There were 515 episodes of myocardial ischemia of which 174 were associated with pain and 341 were asymptomatic. Silent ischemia was significantly more frequent during the first period of monitoring compared to the second (p less than 0.02). Patients who had more silent ischemia on the first occasion also entered more self reports of "emotional upset" (tension, worry, etc.,) in their diaries compared to the second occasion. The level of urinary cortisol was taken as a measure of uncertainty and worry, and was significantly higher on the first occasion (p less than 0.03). Differences in urinary noradrenaline excretion were taken as a measure of subjective stress. Patients who excreted more noradrenaline on the first compared to the second occasion had significantly more silent ischemia (p less than 0.007) and longer total ischemic time (p less than 0.01). We suggest that psychological stress may exacerbate myocardial ischemia which is frequently painless.