Hypertension affects nearly one third of the US population overall, and the prevalence rises sharply with age. In spite of public educational campaigns and professional education programs to encourage blood pressure measurement and control of both systolic and diastolic control to <140/90 mm Hg (or 130/80 mm Hg if diabetic), 43% of treated hypertensives do not achieve the recommended Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure target. Among blacks, 48% are uncontrolled on treatment. The majority of persons classified as poorly controlled hypertensives have mild systolic blood pressure elevation (in the range of 140 to 160 mm Hg). We hypothesized that physician uncertainty regarding the patient's usual blood pressure, as well as uncertainty regarding the extent of medication nonadherence, represent an important barrier to further reductions in the proportion of uncontrolled hypertensives in the United States. Using cluster randomization, 10 primary care clinics (6 from a public health care system and 4 from a private clinic system) were randomized to either the uncertainty reduction intervention condition or to usual care. An average of 68 patients per clinic were recruited to serve as units of observation. Physicians in the 5 intervention clinics were provided with a specially designed study form that included a graph of recent blood pressure measurements in their study patients, a check box to indicate their assessment of the adequacy of the patient's blood pressure control, and a menu of services they could order to aid in patient management. These menu options included 24-hour ambulatory blood pressure monitoring; electronic bottle cap assessment of medication adherence, followed by medication adherence counseling in patients found to be nonadherent; and lifestyle assessment and counseling followed by 24-hour ambulatory blood pressure monitoring. Physicians in the 5 usual practice clinics did not have access to these services but were informed of which patients had been enrolled in the study. Substudies carried out to further characterize the study population and interpret intervention results included ambulatory blood pressure monitoring and electronic bottle cap monitoring in a random subsample of patients at baseline, and audio recording of patient-physician encounters after intervention implementation. The primary study end point was defined as the proportion of patients with controlled blood pressure (<140/90 mm Hg or <130/80 mm Hg if diabetic). Secondary end points include actual measured clinic systolic and diastolic blood pressure, patient physician communication patterns, physician prescribing patient self-reported lifestyle and medication adherence, physician knowledge, attitude and beliefs regarding the utility of intervention tools to achieve blood pressure control, and the cost-effectiveness of the intervention. Six-hundred eighty patients have been randomized, and 675 remain in active follow-up after 1.5 years. Patient closeout will be complete in March 2009. Analyses of the baseline data are in progress. Office-based blood pressure measurement error and bias, as well as physician and patient beliefs about the need for treatment intensification, may be important factors that limit further progress in blood pressure control. This trial will provide data on the extent to which available technologies not widely used in primary care will change physician prescribing behavior and patient adherence to prescribed treatment.