Short-term elevations in ambient fine particulate matter (PM2.5) may increase resting systolic (SBP) and diastolic (DBP) blood pressures, but whether PM2.5 alters hemodynamic responses to orthostatic challenge has not been studied in detail. We repeatedly measured SBP and DBP during supine rest and 1 and 3 minutes after standing among 747 elderly (aged 78.3±5.3 years, mean±SD) participants from a prospective cohort study. We used linear mixed models to assess the association between change in SBP (ΔSBP=standing SBP−supine SBP) and DBP (ΔDBP) on standing and mean PM2.5 levels over the preceding 1 to 28 days, adjusting for meteorologic covariates, temporal trends, and medical history. We observed a 1.4-mm Hg (95% CI: 0.0–2.8 mm Hg; P=0.046) higher ΔSBP and a 0.7-mm Hg (95% CI: 0.0–1.4 mm Hg; P=0.053) higher ΔDBP at 1 minute of standing per interquartile range increase (3.8 μg/m3) in mean PM2.5 levels in the past 7 days. ΔSBP and ΔDBP measured 3 minutes after standing were not associated with PM2.5. Resting DBP (but not SBP or pulse pressure) was positively associated with PM2.5 at longer averaging periods. Responses were more strongly associated with black carbon than sulfate levels. These associations did not differ significantly according to hypertension status, obesity, diabetes mellitus, or sex. These results suggest that ambient particles can increase resting DBP and exaggerate blood pressure responses to postural changes in elderly people. Increased vasoreactivity during posture change may be responsible, in part, for the adverse effect of ambient particles on cardiovascular health.