Cloricromen is a new drug that inhibits platelet aggregation in man and in experimental thrombosis. Twenty patients with a history of atherothrombotic stroke received cloricromen (100 mg, twice daily) for 30 days in order to evaluate its effects on plasma fibrinogen, antithrombin III, and other variables of the haemostatic system. A statistically significant decrease in the prothrombin time (P < 0.01) was found only after 30 days of therapy. This decrease was transient and disappeared 15 days after the end of treatment. No statistically significant changes in plasma fibrinogen levels, antithrombin III, partial thromboplastin time, or platelet count were observed compared with baseline values. No side-effects were reported. This study did not reveal an effect of cloricromen on coagulative variables in patients with cerebrovascular occlusive disease.