Objective: Reported effects of different soy products on blood pressure vary. This systematic review and meta-analysis was performed to clarify the effects of soy isoflavone extract supplements on systolic and diastolic blood pressure (SBP and DBP) in adult humans. Methods: PubMed, CENTRAL, ICHUSHI, and CNKI were searched in June 2009 for relevant randomized placebo-controlled trials. Study data and indicators of methodological validity were independently extracted by two authors using predefined data fields. Meta-analysis was carried out in Review Manager 5.0.22. Results: Searches identified 3740 articles, of which 14 randomized controlled trials (789 participants) were included. Daily ingestion of 25-375 mg soy isoflavones (aglycone equivalents) for 2-24 weeks significantly decreased SBP by 1.92 mmHg (95% confidence interval -3.45 to -0.39; P = 0.01) compared with placebo (heterogeneity P = 0.39, fixed effect model) in adults with normal blood pressure and prehypertension. The effect was not lost on sensitivity analysis. Subgroup analyses suggest greater effects in studies longer than 3 months, in Western populations, at lower doses, and in studies at lower risk of bias. Soy isoflavones did not affect DBP [-0.13 (95% confidence interval -1.03 to 0.78) mmHg, P = 0.78; heterogeneity P = 0.20, fixed effect model]. Conclusion: Soy isoflavone extracts significantly decreased SBP but not DBP in adult humans, and no dose-response relationship was observed. Further studies are needed to address factors related to the observed effects of soy isoflavones on SBP and to verify the effect in hypertensive patients.