Abstract-We measured ambulatory blood pressure (ABP) in 354 participants in the Dietary Approaches to StopHypertension (DASH) Trial to determine the effect of dietary treatment on ABP (24-hour, day and night) and to assess participants' acceptance of and compliance with the ABP monitoring (ABPM) technique. After a 3-week run-in period on a control "typical" American diet, subjects (diastolic blood pressure [BP], 80 to 95 mm Hg; systolic BP, Ͻ160 mm Hg; mean age, 45 years) were randomly assigned to 1 of 3 diets for an 8-week intervention period: a continuation of the control diet; a diet rich in fruits and vegetables; and a "combination" diet that emphasized fruits, vegetables, and low-fat dairy products. We measured ABP at the end of the run-in and intervention periods. Both the fruit/vegetable and combination diets lowered 24-hour ABP significantly compared with the control diet (PϽ0. 0001 for systolic and diastolic pressures on both diets: control diet, Ϫ0.2/ϩ0.1 mm Hg; fruit/vegetable diet, Ϫ3.2/Ϫ1.9 mm Hg; combination diet, Ϫ4.6/Ϫ2.6 mm Hg). The combination diet lowered pressure during both day and night. Hypertensive subjects had a significantly greater response than normotensives to the combination diet (24-hour ABP, Ϫ10.1/Ϫ5.5 versus Ϫ2.3/Ϫ1.6 mm Hg, respectively). After correction for the control diet responses, the magnitude of BP lowering was not significantly different whether measured by ABPM or random-zero sphygmomanometry. Participant acceptance of ABPM was excellent: only 1 participant refused to wear the ABP monitor, and 7 subjects (2%) provided incomplete recordings. These results demonstrate that the DASH combination diet provides significant round-the-clock reduction in BP, especially in hypertensive participants. (Hypertension. 1999;34:472-477.)Key Words: blood pressure monitoring, ambulatory Ⅲ nutrition Ⅲ diet Ⅲ blood pressure T wenty-four-hour ambulatory blood pressure monitoring (ABPM) provides valuable information otherwise unattainable when casual, office-based measurements are used. ABPM has aided our understanding of blood pressure (BP) circadian rhythm and has identified differences in BP rhythms in several population subgroups (eg, less than the expected BP fall occurred during sleep-also called "nondipping"-in blacks and in people with Cushing's syndrome, congestive heart failure, and type II diabetes 1-4 ). Furthermore, in cross-sectional studies, ambulatory blood pressure (ABP) also shows a better correlation with risk of stroke and left ventricular hypertrophy than does casual office measurement, 5-7 although prospective data examining ABP and cardiovascular risk are lacking.ABPM is often used in testing antihypertensive drugs. Compared with standard BP measurements, it simplifies the determination of the time of a drug's peak and trough effects, 8 shows minimal placebo effect, 8,9 and offers less withinsubject test-retest variability than casual office readings. 9,10 There is less experience with ABP in trials of nonpharmacological therapy. The Dietary Approaches to Stop Hyperte...