Hypertension is an extremely common condition, and older adults bear the bulk of morbidity and mortality due to it. The diagnosis of hypertension is almost always based on indirect measurement of arterial blood pressure (BP) through office sphygmomanometry. Although this method is fraught with error, important diagnostic and treatment decisions are routinely made based upon it. In the United States, most hypertension is diagnosed and managed by primary care physicians in their offices. These physicians need improved methods for accounting for BP variability and measurement error as they decide whether or not to recommend potentially lifelong medications for their patients. The objective of this research is to provide rational guidelines, which, if applied in primary care practices, would increase the accuracy of BP determinations, and thereby improve diagnostic accuracy in hypertension. This study will use 24-hour ambulatory blood pressure (ABP) monitoring as a gold-standard to derive guidelines for the optimum determination of usual BP in the medical office. The specific aims of this study are to determine the following in a primary care clinic population of adults aged 45 to 75 years: 1) The number of routine office BP (OBP) measurements, taken 2 per visit, that are required to accurately estimate mean 24-hour ABP; 2) whether OBP measured in standardized fashion in a research clinic and OBP measured by nurses in clinical practice differ significantly from each other in their relationships with mean ABP; 3) the conversion factors necessary to estimate mean ABP from OBP; and 4) comparative direct costs of determining usual BP, comparing OBP measured in these two ways and ABP monitoring, taking into account personnel, time, training and equipment costs. This will be a cross-sectional, observational study involving 75 volunteers, recruited at random from among the patients aged 45 to 75 who attend the Family Practice Center at our institution. The following data will be collected on each participant during 6 visits to the clinic over a 4-6 week period: age, sex, race medical history, height and weight, OBPs measured with both standard and random-zero sphygmomanometers, and ABP monitoring for 24 hours on two separate occasions. The relationships between ABP and OBP will be described using linear regression techniques as well as analyses which focus on the components of variability in BP measurements. The entire project period will be 18 months.