A deficiency of free protein S, known to increase the risk of peripheral venous thrombosis, has not been well described in patients with cerebrovascular disease. In a pilot study of 35 patients with symptomatic cerebrovascular disease, using a qualitative crossed immunoelectrophoresis assay we found eight patients with a free protein S deficiency. A Laurell immunoelectrophoresis assay was used to quantify the percentage of free protein S after removal of the inactive protein S C4b-binding protein complex by precipitation with polyethylene glycol 8000. In a quantitative study of 103 patients with cerebrovascular disease, 21 had a free protein S that was less than 20% of the average normal total protein S concentration (normal range 20-40%); 19 had suffered cerebral infarction and the other two had suffered intracranial hemorrhage. The frequency of free protein S deficiency in this group of stroke patients was far higher than the expected prevalence in the general population.