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Background: Electronic cigarettes (e-cigarettes) have been characterised as significantly less harmful than cigarettes by many health agencies and regulators globally. In this study, we examined to what extent perceived relative harms of e-cigarettes compared to cigarettes have changed in the USA., Methods: We analysed the data from the longitudinal and nationally representative, Population Assessment of Tobacco and Health Study to assess the relative perceived harm of e-cigarettes amongst US adults between 2013 and 2016., Results: The proportion of US adults who correctly perceived e-cigarettes as less harmful than cigarettes decreased each year from 41.1% (CI 40.1-42.1%) in 2013-2014, 31.5% (CI 30.8-32.2%) in 2014-2015 and 25.3% (CI 24.6-26.0%) in 2015-2016. Concurrently, the proportion of US adults who perceived e-cigarettes as equally, or more, harmful than cigarettes increased from 53.7% (CI 52.3-55.1%), 64.9% (CI 63.6-66.2%) to 72.7% (CI 71.5-73.9%) respectively. The proportion of US adults who held negative relative harm perceptions of e-cigarettes increased regardless of current smoking or vaping status by 24.6% and 29.6% respectively within 3 years. In Wave 3, the proportion of current smokers who perceived the relative harm of e-cigarettes as less harmful was lower at 29.3% (CI 28.2-30.4%) compared to current e-cigarette users at 43.5% (CI 40.3-46.7%). Former smokers who used e-cigarettes and believed that they were equally, or more, harmful than cigarettes in 2014-2015 had significantly higher rates of smoking relapse in the following year, 29% and 37% (p < 2.2e-16), respectively, compared to those with positive relative harm perceptions who reported relapse rates of 19%., Conclusions: In this study, the proportion of US adults who incorrectly perceived e-cigarettes as equal to, or more, harmful than cigarettes increased steadily regardless of smoking or vaping status. Current adult smokers appear to be poorly informed about the relative risks of e-cigarettes yet have potentially the most to gain from transitioning to these products. The findings of this study emphasise the urgent need to accurately communicate the reduced relative risk of e-cigarettes compared to continued cigarette smoking and clearly differentiate absolute and relative harms. Further research is required to elucidate why the relative harm of e-cigarettes is misunderstood and continues to deteriorate.