The authors report results of a retrospective pilot study showing a strong association between patients with stroke/TIA and the presence of circulating IgG and IgA antibodies to Chlamydia pneumoniae. These results support the hypothesis that chronic active or persistent infection may play a role in the mechanism of thrombosis. The risk for stroke associated with Chlamydial circulating antibodies appeared to be independent of other risk factors such as diabetes and hypercholesterolemia.