Hereditary deficiency of protein C, protein S or antithrombin III has been associated with an increased incidence of venous thrombosis or pulmonary embolism. The relationship between these deficiencies and the development of arterial thrombosis is a matter of current investigation. We retrospectively studied the occurrence of arterial thrombosis in 92 symptomatic patients belonging to a group of 160 with a confirmed diagnosis of hereditary deficiency of one of the physiologic clotting inhibitors. Seventeen of them experienced at least one arterial thrombotic event. This indicates that about one out of five of the symptomatic patients had experienced arterial thrombosis. The control group consisted of 92 sex and age matched (+/- 5 years) patients with no clotting deficiency who had experienced in the same period at least one episode of deep vein thrombosis or pulmonary embolism. Only one of them had developed arterial thrombosis. Ischemic stroke, myocardial infarction, upper and lower limb arterial thrombosis, and mesenteric artery occlusion occurred regardless of the type of defect taken into account; mean age of about 37.05 +/- 23 years (mean +/- SD). In some cases, arterial thrombosis was fatal. The overall number of venous thrombotic events in the 92 symptomatic patients of this study was much higher than that of arterial thrombosis, with a ratio of 24 to 1. The use of long-term anticoagulant therapy in our group of patients seemed to be able to prevent recurrences of both arterial and venous thrombosis.