To clarify the role of coagulopathy in cerebral infarction (CI) in adults with onset before age 45, we performed comprehensive and repeated hemostatic analyses in 77 consecutive young adult patients with CI. Coagulopathy was found in 24% (9% in older adults, n = 241); inherited coagulopathy was observed in 14 patients, including protein-S deficiency in 6, abnormal plasminogen (genetically confirmed) in 5, and protein-C deficiency in 2. Acquired coagulopathy confined to the acute stage was observed in 6 patients, including siently present anticardiolipin antibody in 3, and transiently present lupus anticoagulant in 2. Of 20 patients who did not have any risk factor for atherosclerosis, acquired coagulopathy was observed in 6. We conclude that protein-S deficiency and abnormal plasminogen are frequent and might be critical causes of CI in younger adults; transient coagulopathy is also a possible cause.