The associations between fine particulate matter (PM2.5) and stroke remain inconsistent. We conducted a time-series study to evaluate emergency room (ER) visits for ischemic and hemorrhagic strokes in relation to PM2.5 and its constituents. Generalized additive models were used to model the counts of daily ER visits for ischemic and hemorrhagic strokes among patients admitted to the National Taiwan University Hospital from 1 January 2004 to 31 August 2008. Exposure variables included PM2.5 and the four constituents, nitrate, sulfate, organic carbon (OC), and elemental carbon (EC). 12,982 ischemic stroke and 3362 hemorrhagic stroke cases were identified during the study period. For hemorrhagic stroke, the strongest relative risks (RRs) of ER visits were 1.19 [95% confidence interval (CI), 1.07-1.32] and 1.08 (95% CI, 1.02-1.15) for an interquartile range (IQR) increase in 3-day average nitrate and EC. For ischemic stroke, increased RRs of ER visits of 1.21 (95% CI, 1.07-1.36) and 1.18 (95% CI, 1.06-1.31) were observed in the warm season for an IQR increase in 3-day average of OC and EC, respectively. PM2.5 and OC were associated with increased RRs of ER visits for ischemic stroke among patients aged 65 years or older and female patients. In conclusion, PM2.5 constituents, rather than PM2.5 mass, are more closely related to ER visits for hemorrhagic stroke. Both PM2.5 mass and its chemical constituents are associated with ER visits for ischemic stroke in the warm season, among patients older than 65 years, and female patients. (C) 2013 Elsevier B.V. All rights reserved.