Previous studies have demonstrated a potential relationship between psychological stress and platelet activation, which may serve as a link between stress and myocardial infarction (MI). However, the possibility that personality traits associated with coronary heart disease may affect platelet activation has not been adequately investigated. The effect of a laboratory stressor (Type A Structured Interview (SI) and speech task) on platelet activation was assessed in 14 stable post-MI patients and 15 age-matched healthy men, using a standardized method of measuring plasma beta-thromboglobulin (BTG) levels. BTG levels were increased after the stressor (average change = 2.0 ng/ml, p = .005). Increases in BTG with stress were related to higher SI ratings of Potential for Hostility (r = .53, p = .004) and Type A behavior (r = .43, p = .02) but not to Cook-Medley-rated hostility scores. Increases in norepinephrine levels and in diastolic blood pressure were nonsignificantly related to increases in BTG levels (ps < .10), whereas increases in epinephrine levels were unrelated. Despite ceasing aspirin and other platelet inhibitors for 10 days before testing, individuals taking platelet inhibitors before the study had less change in BTG with stress (p = .05). However, after statistical adjustment for this factor, SI ratings of Potential for Hostility were still strongly related to increases in BTG with stress (adjusted r = .56, p = .002). Contrary to expectations, healthy men tended to have greater change in BTG with stress than post-MI patients (p = .06). These results indicate that acute stress increases BTG levels and that hostility is related to greater platelet reactivity, independent of any long term effects of platelet inhibition.