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It has been estimated that as many as 4 percent of strokes occurring in people under 45 are the result of some abnormality of the blood clotting process. Protein C is an important regulator of normal clotting, and much evidence has indicated that people who have inherited a deficiency of protein C may suffer venous thrombosis, the occlusion of veins, often in the legs, by blood clots. The data from a series of stroke patients now suggest that this same deficiency may result in strokes as well. To be more precise, the deficiency may result in nonhemorrhagic infarctions in which the circulation to some part of the brain is blocked, presumably by an arterial blood clot. A total of 1,333 patients with acute disease of brain blood vessels were admitted over a period just over four years. Fifty of these occurred in patients under the age of 45. Laboratory investigation revealed that three of these patients had signs of protein C deficiency. To confirm that the condition was inherited, the deficiency was identified in at least one other family member, and the absence of any other conditions which might lead to protein C deficiency was confirmed. Since protein C requires vitamin K for its normal function, it is also important to rule out vitamin K deficiency as a possible contributing factor. In all three cases, no abnormalities of blood coagulation could be found other than the protein C deficiency and no heart abnormalities which might have contributed to clot formation could be found. These cases strongly suggest that protein C deficiency may be a direct contributing factor to the occurrence of stroke in these young patients. Little can be said about the prognosis of strokes arising from this condition. One patient requires support to walk, cannot speak, and cannot use his right arm. The remaining two patients enjoyed almost complete recovery with no residual handicap. (Consumer Summary produced by Reliance Medical Information, Inc.)