BACKGROUND: Hypertension may be poorly controlled because patients do not take their medications (poor adherence) or because providers do not increase medication when appropriate (lack of medication intensification, or ‘clinical inertia’). We examined the prevalence of and relationship between patient adherence and provider treatment intensification. METHODS AND RESULTS: We used a retrospective cohort study of hypertensive patients who had filled prescriptions for 1 or more blood pressure (BP) medications at Veterans’ Affairs (VA) healthcare facilities in a Midwestern VA administrative region. Our sample included all patients who received at least 2 outpatient BP medication refills during 2004 and had 1 or more outpatient primary care visits with an elevated systolic BP >140 but <200 mm Hg or diastolic BP >90 mm Hg during 2005 (n=38,327). For each episode of elevated BP during 2005 (68,610 events), we used electronic pharmacy refill data to examine patients’ BP medication adherence over the prior 12 months and whether providers increased doses or added BP medications (‘intensification’). Multivariate analyses accounted for the clustering of elevated BP events within patients and adjusted for patient age, comorbidities, number of BP medications, encounter systolic BP, and average systolic BP over the prior year. Providers intensified medications in 30% of the 68,610 elevated BP events, with almost no variation in intensification regardless of whether patients had good or poor BP medication adherence. After adjustment, intensification rates were 31% among patients who had ‘gaps’ of <20% (days on which patients should have had medication but no medication was available because medications had not been refilled), 34% among patients with refill gaps of 20% to 59%, and 32% among patients with gaps of 60% or more. CONCLUSIONS: Intensification of medications occurred in fewer than one third of visits in which patients had an elevated BP. Patients' prior medication adherence had little impact on providers' decisions about intensifying medications, even at very high levels of poor adherence. Addressing both patient adherence and provider intensification simultaneously would most likely result in better BP control.