Proportion and causes of brain infarction among young persons may differ between Western world and developing countries, which may require different diagnostic and treatment strategies. We compared a Thai series of 56 hospitalized brain infarct patients younger than 50 years (16% of 357 similar patients from all ages) with a series of 55 Dutch patients (8% of 694 patients). The Thai patients had 12% (95% confidence interval: 3-21) more often a cardioembolic stroke cause, due to a higher frequency of rheumatic heart disease in the Thai series. The percentage of patients with atherothrombotic stroke cause was higher in the Dutch series, whereas the percentage of those with hematologic abnormalities, or those with vasculopathies did not differ. Patients in the Thai series had 17% (95% confidence interval: 7-27) more often an unidentified stroke cause, which was probably related to less extensive ancillary investigations compared with the Dutch series. We conclude that especially centers in developing countries that treat young stroke patients should be supplied with ancillary testing facilities, such as cardiac echo, carotid ultrasound, and some laboratory facilities. Early treatment of infections, especially acute rheumatic fever, could lower the incidence of stroke in the young.