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Background: Acute respiratory distress syndrome (ARDS) occurs commonly in intensive care units. The reported mortality rates in studies evaluating ARDS are highly variable. Objective: To investigate mortality rates due to ARDS from before the 2009 H1N1 influenza pandemic began until the start of coronavirus disease 2019 (COVID-19) pandemic. Design: We performed a systematic search and then ran a proportional meta-analysis for mortality. We ran our analysis in three ways: for randomised controlled trials only, for observational studies only, and for randomised controlled trials and observational studies combined. Data sources: MEDLINE and Embase, using a highly sensitive criterion and limiting the search to studies published from January 2009 to December 2019. Review methods: Two of us independently screened titles and abstracts to first identify studies and then complete full text reviews of selected studies. We assessed risk of bias using the Cochrane RoB-2 (a risk-of-bias tool for randomised trials) and the Cochrane ROBINS-1 (a risk-of-bias tool for non-randomised studies of interventions). Results: We screened 5844 citations, of which 102 fully met our inclusion criteria. These included 34 randomised controlled trials and 68 observational studies, with a total of 24 158 patients. The weighted pooled mortality rate for all 102 studies published from 2009 to 2019 was 39.4% (95% CI, 37.0-41.8%). Mortality was higher in observational studies compared with randomised controlled trials (41.8% [95% CI, 38.9-44.8%] v 34.5% [95% CI, 30.6-38.5%]; P = 0.005). Conclusions: Over the past decade, mortality rates due to ARDS were high. There is a clear distinction between mortality in observational studies and in randomised controlled trials. Future studies need to report mortality for different ARDS phenotypes and closely adhere to evidence-based medicine. PROSPERO registration: CRD42020149712 (April 2020)., Competing Interests: Abhijit Duggal is a member of the Steering Committee for ALung Technologies, not related to this publication. All other authors declare that they do not have any potential conflict of interest in relation to this manuscript., (© 2022 College of Intensive Care Medicine of Australia and New Zealand.)