Endothelial damage, platelet hyperactivity and other changes of blood coagulation may play a role in the vascular complications of essential hypertension. Undesirable changes of haemostasis induced by some anti-hypertensive drugs can encourage the acceleration of atherogenesis. Therefore, the effect of angiotensin-converting enzyme (ACE)-inhibitors on haemostasis is of interest. The therapeutic dose of perindopril was previously shown to reduce platelet aggregation. In the present study, selected parameters of haemostasis were investigated in 23 patients with first and second stage of non-treated essential hypertension. The measurements were carried out before therapy, after 1 week of placebo administration, and after 1 week and after 1 month of ACE-inhibitor perindopril therapy in a once-daily dose of 4 mg. Plasma prothrombin time, activated partial thromboplastin time, fibrinogen level, plasminogen and antithrombin III activities, protein C and free protein S antigens, total fibrinolytic activity as well as fibrin monomers and D-dimers were assayed. There were no significant changes in any haemostasis variables investigated following placebo administration or perindopril therapy. On the basis of this study, no unfavourable effects on haemostasis induced by this therapy were found. The platelet-inhibitory effect of perindopril, without any harmful effects on coagulation or fibrinolytic activity and coagulation inhibitors, is desirable in the new approach to hypertension treatment. These properties of perindopril may be important in terms of the beneficial role of anti-hypertensive drugs in cardiovascular morbidity.