Over the past decade there has been a marked increase in the utilization of 24-h ambulatory blood pressure (BP) monitoring to assess new drugs for hypertension, beginning with the first and second phases of drug development (studies that establish the range of doses in patients with hypertension), continuing to the fourth phase, in which the drugs approved for the marketplace are tested in comparison to other registered drugs or in a special population, such as the elderly or diabetic patient with hypertension.