Background: To investigate the effect of COX-2 polymorphism and its product, prostaglandin E2 (PGE2), on stroke risk in an endemic area for Chagas disease. In a separate cohort, to investigate the effect of COX-2 polymorphisms on the total burden of cerebral white matter disease. Methods: Cases were outpatients with ischemic stroke; controls were stroke-free subjects from 2 outpatient clinics (heart failure and caregivers of a movement disorders clinic). We extracted DNA from total blood to investigate the rs20417 COX-2 polymorphism. Serologic tests (Enzime-linked immunosorbent assay) were performed to confirm Trypanosoma cruzi infection and to quantify PGE2 levels. In the Boston cohort, white matter hyperintensity volume (WMHv) was quantified on the admission brain magnetic resonance images of subjects with ischemic stroke, who also donated DNA for the COX-2 gene region analysis. Results: We studied 44 patients with stroke and 96 controls (46 with heart failure and 50 caregivers) in the Brazilian cohort; and 788 stroke patients (302 cardioembolic and 486 noncardioembolic) in the Boston cohort. In the Brazilian cohort, rs20417 polymorphism was associated with both stroke (P 5 5 3 1026 ) and decreased PGE2 levels (P 5 4 3 1025 ); similarly, Chagas was associated with stroke (P 5 4 3 1023 ) and decreased PGE2 levels (P 5 7 3 1023 ). In the Boston cohort, rs20417 polymorphism was associated with increased WMHv among noncardioembolic (P 5.037), but not among cardioembolic stroke patients. Conclusions: Variation in COX-2 gene is associated with both symptomatic and silent brain cerebrovascular disease. This candidate gene region should be tested in population-based samples