Clinical decisions are ideally based on evidence generated from multiple randomized controlled trials (RCTs) evaluating clinical outcomes, but historically, few clinical guideline recommendations have been based entirely on this type of evidence. To determine the class and level of evidence (LOE) supporting current major cardiovascular society guideline recommendations, and changes in LOE over time. Current American College of Cardiology/American Heart Association (ACC/AHA) and European Society of Cardiology (ESC) clinical guideline documents (2008-2018), as identified on cardiovascular society websites, and immediate predecessors to these guideline documents (1999-2014), as referenced in current guideline documents. Comprehensive guideline documents including recommendations organized by class and LOE. The number of recommendations and the distribution of LOE (A supported by data from multiple RCTs or a single, large RCT, B supported by data from observational studies or a single RCT, and C supported by expert opinion only) were determined for each guideline document. The proportion of guideline recommendations supported by evidence from multiple RCTs (LOE A). Across 26 current ACC/AHA guidelines (2930 recommendations; median, 121 recommendations per guideline 25th-75th percentiles, 76-155), 248 recommendations (8.5%) were classified as LOE A, 1465 (50.0%) as LOE B, and 1217 (41.5%) as LOE C. The median proportion of LOE A recommendations was 7.9% (25th-75th percentiles, 0.9%-15.2%). Across 25 current ESC guideline documents (3399 recommendations; median, 130 recommendations per guideline 25th-75th percentiles, 111-154), 484 recommendations (14.2%) were classified as LOE A, 1053 (31.0%) as LOE B, and 1862 (54.8%) as LOE C. When comparing current guidelines with prior versions, the proportion of recommendations that were LOE A did not increase in either ACC/AHA (median, 9.0% current vs 11.7% prior) or ESC guidelines (median, 15.1% current vs 17.6% prior). Among recommendations in major cardiovascular society guidelines, only a small percentage were supported by evidence from multiple RCTs or a single, large RCT. This pattern does not appear to have meaningfully improved from 2008 to 2018.