Background-Observational studies demonstrate that communities of low socioeconomic status have higher blood pressure and worse cardiovascular outcomes. Yet, whether the clinical outcomes resulting from antihypertensive therapy vary by socioeconomic context in a randomized clinical trial, in which participants are treated under a standard protocol, is unknown.Methods and Results-We used data from ALLHAT (Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial) to study the effect of socioeconomic context, defined as the county-level median household income, of study sites. We stratified sites into income quintiles and compared characteristics, blood pressure control, and cardiovascular outcomes among ALLHAT participants in the lowest- and highest-income quintiles. Among 27 862 qualifying participants, 2169 (7.8%) received care in the lowest-income sites (quintile 1) and 10 458 (37.6%) received care in the highest-income sites (quintile 5). Participants in quintile 1 were more likely to be women, to be black, to be Hispanic, to have fewer years of education, to live in the South, and to have fewer cardiovascular risk factors. After adjusting for baseline demographic and clinical characteristics, quintile 1 participants were less likely to achieve blood pressure control (<140/90 mm Hg) (odds ratio, 0.48; 95% CI, 0.37-0.63) and had greater all-cause mortality (hazard ratio [HR], 1.25; 95% CI, 1.10-1.41), heart failure hospitalizations/mortality (HR, 1.26; 95% CI, 1.03-1.55), and end-stage renal disease (HR, 1.86; 95% CI, 1.26-2.73), but lower angina hospitalizations (HR, 0.70; 95% CI, 0.59-0.83) and coronary revascularizations (HR, 0.71; 95% CI, 0.57-0.89).Conclusions-Despite standardized treatment protocols, ALLHAT participants in the lowest-income sites experienced poorer blood pressure control and worse outcomes for some adverse cardiovascular events, emphasizing the importance of measuring and addressing socioeconomic context.