This randomized control trial assessed the post-intervention and 18-month follow-up effects of a 6-month dietary approaches to stop hypertension (DASH)-focused behavioral nutrition intervention, initiated in clinic with subsequent telephone and mail contact, on blood pressure (BP) and endothelial function in adolescents with elevated BP. Adolescents (n=159) 11 to 18 years of age with newly diagnosed elevated BP or stage 1 hypertension treated at a hospital-based clinic were randomized. DASH participants received a take-home manual plus 2 face-to-face counseling sessions at baseline and 3 months with a dietitian regarding the DASH diet, 6 monthly mailings, and 8 weekly and then 7 biweekly telephone calls focused on behavioral strategies to promote DASH adherence. Routine care participants received nutrition counseling with a dietitian consistent with pediatric guidelines established by the National High Blood Pressure Education Program. Outcomes, measured pre- and post-intervention and at 18-months follow-up, included change in BP, change in brachial artery flow-mediated dilation, and change in DASH score based on 3-day diet recalls. Adolescents in DASH versus routine care had a greater improvement in systolic BP (–2.7 mm Hg, P = 0.03, –0.3 z-score, P =0.03), flow-mediated dilation (2.5%, P =0.05), and DASH score (13.3 points, P <0.0001) from baseline to post-treatment and a greater improvement in flow-mediated dilation (3.1%, P =0.03) and DASH score (7.4 points, P =0.01) to 18 months. The DASH intervention proved more effective than routine care in initial systolic BP improvement and longer term improvement in endothelial function and diet quality in adolescents with elevated BP and hypertension. Registration: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT00585832.