Deficiency of protein S has been associated with an increased risk of thrombotic disease as already shown for protein C deficiency. Deficiencies of any of these two proteins predispose to venous thrombosis but have been only rarely associated with arterial thrombosis. In this study we describe a case of severe cerebral arterial thrombosis in a 44-year old woman with protein S deficiency. The defect was characterized by moderately reduced levels of total and markedly reduced levels of free protein S. C4b-bp level was normal. Protein C, AT III and routine coagulation tests were within the normal limits. In her family two other members showed the same defect. All the affected members had venous thrombotic manifestations, two of them at a relatively young age. No other risk factors for thrombotic episodes were present in the family members. The patient reported was treated with ASA and dipyridamole and so far there were no relapses.