The 2017 American College of Cardiology/American Heart Association guideline defines hypertension as a blood pressure ≥130/80 mm Hg, whereas the 2018 European Society of Cardiology (ESC) and 2019 National Institute for Health and Care Excellence (NICE) guidelines use a ≥140/90 mm Hg threshold. Our objective was to study the associations between isolated diastolic hypertension (IDH), diagnosed using these 2 blood pressure thresholds, and cardiovascular disease (CVD) in a large cohort of UK adults. We analyzed data from UK Biobank, which enrolled participants between 2006 and 2010 with follow-up through March 2019. We excluded persons with systolic hypertension or baseline CVD. We defined incident CVD as a composite of nonfatal myocardial infarction, nonfatal stroke, or cardiovascular death. We used Cox regression to quantify associations between IDH and CVD, as well as the individual outcomes included in the composite outcome. We studied 151 831 participants with normal systolic blood pressure (mean age 54 years, 40% male). Overall, 24.5% had IDH by the American College of Cardiology/American Heart Association definition compared with 6% by the ESC/NICE definition. Compared with normal diastolic blood pressure, IDH by the American College of Cardiology/American Heart Association definition was not significantly associated with CVD risk (hazard ratio, 1.08 [95% CI, 0.98-1.18]) whereas IDH by the ESC/NICE definition was significantly associated with a modest increase in CVD (hazard ratio, 1.15 [95% CI, 1.04-1.29]). Similar results were found by sex and among participants not taking baseline antihypertensives. Furthermore, neither IDH definition was associated with the individual outcomes of nonfatal myocardial infarction or stroke. In conclusion, the proportion of UK Biobank participants with IDH was significantly higher by the American College of Cardiology/American Heart Association definition compared with the ESC/NICE definitions; however, only the ESC/NICE definition was statistically associated with increased CVD risk.