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The objective of this review was to evaluate the effectiveness of relaxation to lower high blood pressure. We searched electronic bibliographic databases and grey literature to identify randomized controlled trials comparing relaxation therapies with no active treatment or sham therapy, enrolling adult participants with raised systolic blood pressure (SBP) ≥ 140 mm Hg or diastolic blood pressure (DBP) ≥ 85 mm Hg and follow-up ≥ 8 weeks. Twenty-five trials with up to 5 years follow-up, assessing 1198 participants, met our inclusion criteria and were meta-analysed. Overall, relaxation resulted in small, statistically significant reductions in SBP (mean difference: −5.5 mm Hg, 95% CI: −8.2 to −2.8) and DBP (mean difference: −3.5 mm Hg, 95% CI: −5.3 to −1.6) compared to the control. Substantial heterogeneity between trials (I2 > 70%) was not explained by duration of follow-up, type of control, type of relaxation therapy or baseline blood pressure. The 9 trials that reported blinding of outcome assessors found a non-significant net reduction in blood pressure (SBP mean difference: −3.2 mm Hg, 95% CI: −7.7 to 1.4) associated with relaxation, as did the 15 trials comparing relaxation with sham therapy (SBP mean difference: −3.5 mm Hg, 95% CI: −7.1 to 0.2). Adequate randomization was confirmed in only seven trials and concealment of allocation in only one. In view of the poor quality of the included trials and unexplained variation between trials, the evidence in favour of a causal association between relaxation and blood pressure reduction is weak. Some of the apparent benefit of relaxation was probably due to aspects of treatment unrelated to relaxation.