Chlamydia pneumoniae has been linked with increased risk of cardiovascular disease, but data on stroke are sparse. We examined whether seropositivity to Chlamydia pneumoniae was associated with the risk of ischemic stroke in a nested case-control study. Data on Chlamydia pneumoniae serology, lifestyle factors, and medical history were obtained at baseline. Verified cases (n = 254) were compared with gender- and age-matched controls (n = 254). Positive IgA (> or = 1:16) or IgG (> or = 1:64) titers were associated with an increased risk of acute ischemic stroke, i.e. adjusted odds ratios (ORs) were 1.54 (95% confidence interval, CI: 0.96-2.47) and 1.28 (95% CI: 0.83-1.95). The adjusted OR was 1.77 (95% CI: 1.04-3.00) when both titers were elevated. The highest point estimates were seen for ischemic stroke due to large-artery atherosclerosis, adjusted OR: 6.32 (95% CI: 0.76-52.61) (IgG (> or = 1:64)). No clear associations were found for other types of ischemic stroke. The strength of the association varied depending on gender and the chosen cut-off values for the antibody titers. These results partly support the hypothesis that serologic evidence of Chlamydia pneumoniae infection may be associated with an increased risk of ischemic stroke. However, the risk may differ according to gender, subtype of ischemic stroke, and cut-off value of antibody titers.