Plasma B beta 15-42 and fibrinopeptide A (FPA) concentrations, which are respectively indicators of plasmin and thrombin in vivo activity, were measured in 46 patients with ischemic arterial disease without signs of acute thrombosis. In the group as a whole, an increase in both B beta 15-42 and FPA was found. When the patients were divided in two groups on the basis of their reversible (transitory ischemic attacks and unstable angina) or irreversible (stroke and myocardial infarction) ischemic episodes, the levels of B beta 15-42 were significantly elevated only in the former group when compared to controls (p less than 0.01). In the latter group we found significantly increased levels of FPA with respect to both controls (p less than 0.01) and patients with reversible and transient ischemic episodes (p less than 0.05). Moreover, the B beta 15-42/FPA ratio was significantly lower in patients with irreversible ischemic episodes than in controls (p less than 0.01) and patients with transient ischemic episodes (p less than 0.01), while no difference was found between the latter group and controls, although FPA and B beta 15-42 were significantly higher. These results suggest that in patients with transient and reversible ischemic episodes fibrinolytic activity is able to counterbalance an increased thrombin activity, while this does not appear to occur in patients with irreversible ischemic episodes.