Aa a plasma marker of an endothelial abnormality, the serum activity of angiotensin-converting enzyme (ACE) was investigated at rest and after stimulation either by local venostasis or infusion of an analogue of lysine-vasopressin (desmopressin acetate). Desmopressin acetate did not induce any significant change in ACE, in contrast to the effect of venostasis. Searching for an endothelial abnormality implicated in the genesis of deep vein thrombosis, we used the local venostasis test in patients affected by recurrent deep vein thrombosis. Patients, divided in three groups (group I, documented history of recurrent deep vein thrombosis; group II, only one deep vein thrombosis or recurrent superficial venous thrombosis; group III, history of arterial thromboembolism), and controls were screened for basal and stimulated levels of serum ACE, together with fibrinolytic activity and von Willebrand factor level. Two types of abnormalities of serum ACE activity were found: low basal level in group I, and low response to venostasis in groups I and III; group II did not differ from controls. Measures of fibrinolytic and ACE activities are not redundant because the two types of ACE abnormalities were not individually encountered in the same patients and were independent from abnormalities of the fibrinolytic system. These findings suggest that an endothelial lesion could participate in the pathogenesis of some forms of recurrent deep vein thrombosis and support interest in the measurement of serum ACE to discriminate some patients at high risk of deep vein thrombosis.