en تخصصي Special پژوهشي Research Objective: Hypertension is a common component of metabolic syndrome (MetS). Some studies showed that L-arginine reduced the blood pressure (BP) .Therefore this study was designed to evaluate the effect of L-arginine supplementation on BP in MetS.<br> Materials and Methods: In this randomized, double-blind, placebo-controlled trials, 60 patients with MetS were randomly divided into two groups: the L-arginine-supplemented group (AG), who received 5 gr of L-arginine daily and placebo group (PG). Systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), pulse pressure (PP) were measured before and after the intervention and compared.<br> Results: At the end of the study in the AG significant reductions were observed in the SBP, DBP and MAP (P-value:0.001).The comparison of changes in variables between the AG and PG indicated that in the AG, patients had a significantly greater decrease in the SBP (-11.25&plusmn;10.93 vs. -2.67&plusmn;6.15 mmHg: P-value:0.001), DBP(-5.71&plusmn;7.16 vs. -1.25&plusmn;7.02 mmHg: p-value: 0.02) and MAP (-7.55&plusmn;5.61 vs.-1.72&plusmn;5.16 mmHg, P-value: 0.001). The PP decreased in both groups but a significant decrease compared with baseline in the AG was observed (-5.53&plusmn;13.83 vs. -1.42&plusmn;9.21 mmHg: P-value: 0.19).<br> Conclusion: The findings of the present study showed that 5 gr/d L- arginine supplementation for 12 weeks in MetS resulted in significant decrease in the SBP, DBP and MAP but had no significant effect on the PP. Metabolic syndrome, L-arginine, Blood pressure 1 10 http://ijdo.ssu.ac.ir/browse.php?a_code=A-10-30-189&slc_lang=en&sid=1 Davood Bahrami No Department of Nutrition, School of Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran Hassan Mozaffari-Khosravi Yes Department of Nutrition, School of Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran