Therapeutic inertia (TI), defined as the providers' failure to increase therapy when treatment goals are unmet, contributes to the high prevalence of uncontrolled hypertension (> or =140/90 mm Hg), but the quantitative impact is unknown. To address this gap, a retrospective cohort study was conducted on 7253 hypertensives that had > or =4 visits and > or =1 elevated blood pressure (BP) in 2003. A 1-year TI score was calculated for each patient as the difference between expected and observed medication change rates with higher scores reflecting greater TI. Antihypertensive therapy was increased on 13.1% of visits with uncontrolled BP. Systolic BP decreased in patients in the lowest quintile of the TI score but increased in those in the highest quintile (-6.8+/-0.5 versus +1.8+/-0.6 mm Hg; P<0.001). Individuals in the lowest TI quintile were &33 times more likely to have their BP controlled at the last visit than those in highest quintile (odds ratio, 32.7; 95% CI, 25.1 to 42.6; P<0.0001). By multivariable analysis, TI accounted for &19% of the variance in BP control. If TI scores were decreased &50%, that is, increasing medication dosages on &30% of visits, BP control would increase from the observed 45.1% to a projected 65.9% in 1 year. This study confirms the high rate of TI in uncontrolled hypertensive subjects. TI has a major impact on BP control in hypertensive subjects receiving regular care. Reducing TI is critical in attaining the Healthy People 2010 goal of controlling hypertension in 50% of all patients.