With the use of antihypertensive medication increasing steadily over the last 25 years, treatment for hypertension is now common. Given the high prevalence of treatment for hypertension, researchers can no longer assume that participants in epidemiologic studies represent natural blood pressure measures, as opposed to treated blood pressure measures. The effect of this high prevalence of treatment, however, has not been fully investigated. Although it might seem that studies of "natural history" are no longer relevant, great interest currently exists in establishing both environmental and genetic determinants of blood pressure. Additionally, there currently is a renewed interest in refining treatment guidelines. For example, the need to account for absolute as well as relative risk has been hotly debated in recent years. The need will continue to exist, therefore, to analyze observational data sets to meet the evolving interest in the determinants of blood pressure and to meet the requirements for more refined risk estimates.