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Background The American College of Cardiology and American Heart Association periodically revise clinical practice guidelines. We evaluated changes in the evidence underlying guidelines published over a 10‐year period. Methods and Results Thirty‐five American College of Cardiology/American Heart Association guidelines were divided into 2 time periods: 2008 to 2012 and 2013 to 2017. Guidelines were categorized into the following topic areas: arrhythmias, prevention, acute and stable ischemia, heart failure, valvular heart disease, and vascular medicine. Changes in recommendations were assessed for each topic area. American College of Cardiology/American Heart Association designated class of recommendation as level I, II , or III (I represented “strongly recommended”) and levels of evidence ( LOE ) as A, C, or C (A represented “highest quality”). The median number of recommendations per each topic area was 281 (198–536, interquartile range) in 2008 to 2012 versus 247 (190–451.3, interquartile range) in 2013 to 2017. The median proportion of class of recommendation I was 49.3% and 44.4% in the 2 time periods, 38.0% and 44.5% for class of recommendation II , and 12.5% and 11.2% for class of recommendation III . Median proportions for LOE A were 15.7% and 14.1%, 41.0% and 52.8% for LOE B, and 46.9% and 32.5% for LOE C. The decrease in the proportion of LOE C was highest in heart failure (24.8%), valvular heart disease (22.3%), and arrhythmia (19.2%). An increase in the proportion of LOE B was observed for these same areas: 31.8%, 23.8%, and 19.2%, respectively. Conclusions There has been a decrease in American College of Cardiology/American Heart Association guidelines recommendations, driven by removal of recommendations based on lower quality of evidence, although there was no corresponding increase in the highest quality of evidence.