We measured ambulatory blood pressure (ABP) in 354 participants in the Dietary Approaches to Stop Hypertension (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, <160 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 (P<0. 0001 for systolic and diastolic pressures on both diets: control diet, -0.2/+0.1 mm Hg; fruit/vegetable diet, -3.2/-1.9 mm Hg; combination diet, -4.6/-2. 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, -10.1/-5.5 versus -2.3/-1.6 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.