Recent studies have suggested a link between exposure to ambient particulate matter <2.5 μm in diameter (PM_2.5) and adverse cardiovascular outcomes. The objective of this study was to examine the effects of differing community-level exposure to PM_2.5 on daily measures of blood pressure (BP) among an adult population. During the period May 2002 through April 2003, BP was examined at 2 time points for 347 adults residing in 3 distinct communities of Detroit, Michigan. Exposure to PM_2.5 was assessed in each community during this period, along with multivariate associations between PM_2.5 and BP. In models combining all 3 of the communities, PM_2.5 was significantly associated with systolic blood pressure; a 10-μg/m³ increase in daily PM_2.5 was associated with a 3.2-mm Hg increase in systolic blood pressure (P=0.05). However, in models that added a location interaction, larger effects were observed for systolic blood pressure within the community with highest PM_2.5 levels; a 10-μg/m³ increase in daily PM_2.5 was associated with a 8.6-mm Hg increase in systolic blood pressure (P=0.01). We also found young age (<55 years) and not taking BP medications to be significant predictors of increased BP effects. Among those taking BP medications, the PM_2.5 effect on BP appeared to be mitigated, partially explaining the age effect, because those participants <55 years of age were less likely to take BP medications. Short-term increases in exposure to ambient PM_2.5 are associated with acute increases in BP in adults, especially within communities with elevated levels of exposure.