Despite improvements in blood pressure (BP) control, a substantial percentage of patients do not achieve target. The relative importance of determinants of poor BP control is unclear. Therefore, the authors conducted a post hoc exploratory analysis to assess determinants of BP control. Data were collected in 45 general practices, which enrolled patients with uncontrolled hypertension. Antihypertensive medication changes throughout the 6-month follow-up period were documented. Baseline and 6-month BPs were recorded. Of the 2030 patients analyzed, 320 had diabetes. Overall, 42% of patients did not achieve BP control. In multivariate analysis, failure to intensify therapy was identified as a significant independent predictor of lesser BP reduction. Of patients unable to reach target after 6 months, only 25% were prescribed ≥3 drugs. Patients with diabetes were significantly less likely to reach target than those without (26% vs 64%, P<.001). Antihypertensive therapy prescribed to patients with diabetes was only marginally more intensive than to those without. In patients with hypertension, whether with or without coexisting diabetes, poor BP control appears to be at least partially due to failure to uptitrate antihypertensive therapy. Clinical inertia is likely an important barrier to BP control. J Clin Hypertens (Greenwich). 2011;13:73–80. © 2010 Wiley Periodicals, Inc.