Data for trends in blood pressure are needed to understand the effects of its dietary, lifestyle, and pharmacological determinants; set intervention priorities; and evaluate national programmes. However, few worldwide analyses of trends in blood pressure have been done. We estimated worldwide trends in population mean systolic blood pressure (SBP). We estimated trends and their uncertainties in mean SBP for adults 25 years and older in 199 countries and territories. We obtained data from published and unpublished health examination surveys and epidemiological studies (786 country-years and 5·4 million participants). For each sex, we used a Bayesian hierarchical model to estimate mean SBP by age, country, and year, accounting for whether a study was nationally representative. In 2008, age-standardised mean SBP worldwide was 128·1 mm Hg (95% uncertainty interval 126·7-129·4) in men and 124·4 mm Hg (123·0-125·9) in women. Globally, between 1980 and 2008, SBP decreased by 0·8 mm Hg per decade (-0·4 to 2·2, posterior probability of being a true decline=0·90) in men and 1·0 mm Hg per decade (-0·3 to 2·3, posterior probability=0·93) in women. Female SBP decreased by 3·5 mm Hg or more per decade in western Europe and Australasia (posterior probabilities ≥0·999). Male SBP fell most in high-income North America, by 2·8 mm Hg per decade (1·3-4·5, posterior probability >0·999), followed by Australasia and western Europe where it decreased by more than 2·0 mm Hg per decade (posterior probabilities >0·98). SBP rose in Oceania, east Africa, and south and southeast Asia for both sexes, and in west Africa for women, with the increases ranging 0·8-1·6 mm Hg per decade in men (posterior probabilities 0·72-0·91) and 1·0-2·7 mm Hg per decade for women (posterior probabilities 0·75-0·98). Female SBP was highest in some east and west African countries, with means of 135 mm Hg or greater. Male SBP was highest in Baltic and east and west African countries, where mean SBP reached 138 mm Hg or more. Men and women in western Europe had the highest SBP in high-income regions. On average, global population SBP decreased slightly since 1980, but trends varied significantly across regions and countries. SBP is currently highest in low-income and middle-income countries. Effective population-based and personal interventions should be targeted towards low-income and middle-income countries. Funding Bill & Melinda Gates Foundation and WHO. Copyright © 2011 Elsevier Ltd. All rights reserved.