Garlic has been suggested to lower blood pressure; however, studies evaluating this parameter have provided conflicting results. To examine the effect of garlic on blood pressure in patients with and without elevated systolic blood pressure (SPB) through meta-analyses of randomized controlled trials. A systematic search of MEDLINE, CINAHL, and the Cochrane Central Register of Controlled Trials was conducted to identify randomized controlled trials in humans evaluating garlic's effect on blood pressure. All databases were searched from their inception through June 26, 2008, using the key words garlic, Allium sativum, and allicin. A manual search of published literature was used to identify additional relevant studies. To be included in the analysis, studies must have been written in English or German and reported endpoints of SBP or diastolic blood pressure (DBP). Studies whose population had a mean baseline SBP greater than 140 mm Hg were evaluated separately from those whose population had lower baseline blood pressures. Garlic's effect on SBP and DBP was treated as a continuous variable and weighted mean differences were calculated using a random-effects model. Ten trials were included in the analysis; 3 of these had patients with elevated SBP. Garlic reduced SBP by 16.3 mm Hg (95% CI 6.2 to 26.5) and DBP by 9.3 mm Hg (95% CI 5.3 to 13.3) compared with placebo in patients with elevated SBP. However, the use of garlic did not reduce SBP or DBP in patients without elevated SBP. There was only a minor degree of heterogeneity in the analyses and publication bias did not appear to influence the results. This meta-analysis suggests that garlic is associated with blood pressure reductions in patients with an elevated SBP although not in those without elevated SBP. Future research should focus on the impact of garlic on clinical events and the assessment of the long-term risk of harm.