While many studies have demonstrated the pathogenetic role of inherited deficiency of natural clotting inhibitors in patients in the development of deep vein thrombosis of lower limbs, no data are available on the prevalence of these abnormalities in patients with upper vein thrombosis. In this study, antithrombin III, protein C, protein S, plasminogen, resistance to activated protein C and factor V Leiden mutation were assayed in 27 consecutive patients with thrombosis of upper extremities. Only two patients (7.4%) showed a congenital defect (one patient with deficiency of protein C, confirmed by family study, and one patient with factor V Leiden mutation). Anticardiolipin antibodies were also measured and four patients (14.8%) had increased levels, confirmed on a subsequent occasion 3 months later. Eighteen out of 27 (67%) had a predisposing or triggering factor, thus emphasizing the role of physical stress in the development of upper vein thrombosis. At variance with what is observed in deep vein thrombosis of the lower limbs, inherited clotting abnormalities seem to be rarely responsible for upper vein thrombosis, whereas anticardiolipin antibodies and cancer are implicated in a significant proportion of cases.