Background Only one third of patients recommended intensified treatment by the 2017 American College of Cardiology/American Heart Association (ACC/AHA) guideline for high blood pressure would have been eligible for the clinical trials on which recommendations were largely based. We sought to identify characteristics of adults who would have been trial‐ineligible in order to inform clinical practice and research priorities. Methods and Results We examined the proportion of adults diagnosed with hypertension who met trial inclusion and exclusion criteria, stratified by age, diabetes mellitus status, and guideline recommendations in a cross‐sectional study of the National Health and Nutrition Examination Survey, 2013–2016. Of the 107.7 million adults (95% CI, 99.3–116.0 million) classified as having hypertension by the ACC/AHA guideline, 23.1% (95% CI, 20.8%–25.5%) were below the target blood pressure of 130/80 mm Hg, 22.2% (95% CI, 20.1%–24.4%) would be recommended nonpharmacologic treatment, and 54.6% (95% CI, 52.5%–56.7%) would be recommended additional pharmacotherapy. Only 20.6% (95% CI, 18.8%–22.4%) of adults with hypertension would be trial‐eligible. The majority of adults <50 years were excluded because of low cardiovascular risk and lack of access to primary care. The majority of adults aged ≥70 years were excluded because of multimorbidity and limited life expectancy. Reasons for trial exclusion were similar for patients with and without diabetes mellitus. Conclusions Intensive blood pressure treatment trials were not representative of many younger adults with low cardiovascular risk and older adults with multimorbidity who are now recommended more intensive blood pressure goals.