The overall objectives of this proposal are to study the effect of measurement errors by the standard mercury sphygmomanometer (SMS) on the management of hypertension and to determine if more accurate but more expensive, alternative technologies such as random zero sphygmomanometer (RZS), electronic automatic recorders (AR) and 24 hour ambulatory blood pressure monitors (ABPM) lead to improved clinical decision making and cost effective blood pressure management. Ambulatory hypertensive patients followed in the general internal medicine clinic at the University of Wisconsin Hospital and Clinics will be studied with concurrent BP measurements by SMS, RZS, AR and ABPM. The specific aims are: 1) to determine the frequency of misclassification of hypertension using the SMS, RZS and AR in comparison to the ABPM as the reference standard; 2) to assess the impact of misclassification of hypertension status due to measurement errors with the SMS on treatment decisions of clinic physicians; 3) to evaluate the feasibility and effectiveness of an intervention strategy to reduce inappropriate treatment decisions due to errors with SMS by providing physicians BP values obtained with the newer methods; 4) to determine if more accurate measurements with the improved technology lead to more appropriate management decisions and better BP control; and 5) to analyze the cost effectiveness of different BP measurement techniques in classifying BP status. The ultimate objective of the study is to determine if there is justification for the widespread use and high investment cost of newer technologies in the clinical setting. The results of the study will contribute to improved decision making in the management of patients with established hypertension, who are subject to life-long treatment with antihypertensive therapy. The study has important societal benefits given the high prevalence of hypertension in the general population. Directors of outpatient clinics, and all personnel involved with the care and decision making of hypertensive patients would benefit from the results of the study. The study will consist of 2 phases: an initial, observational phase and a sequential, interventional phase. The initial, pilot phase will provide the baseline information of hypertension misclassification by SMS and its impact on clinicians' treatment decisions in managing BP. The appropriateness of treatment decisions based on the SMS measurements will be compared to the management appropriate for the BP as measured by the ABPM. In the interventional phase, readings obtained with more advanced methods (RZS, AR, and ABPM) will be provided to the clinicians. The appropriateness of BP management after the intervention will be compared to that of the baseline. A formal cost effectiveness analysis will be performed by evaluating the medical care and patient costs associated with treatment of hypertension and its sequelae relative to clinical impact of the four different methods of measuring BP.