Protein S is found in two forms in plasma; as free and functionally active protein S, and complexed to C4b-binding protein. Patients with nephrotic syndrome are at risk for arterial and venous thrombosis at various localizations, and acquired protein S deficiency due to the selective urinary loss of the free form may be a risk factor for the development of thromboembolic complications. We report a case of cerebral arterial thrombosis associated with decreased level of free protein S antigen (44%) in a 39-year-old female patient with nephrotic syndrome.