Background The 2017 American College of Cardiology (ACC)/American Heart Association (AHA) guidelines lowered the threshold of blood pressure (BP) for hypertension to 130/80 mm Hg. However, the clinical significance of isolated diastolic hypertension (IDH) according to the cutoff value of the 2017 ACC/AHA guidelines was uncertain. Methods and Results We analyzed the claims database of Japan Medical Data Center (a nationwide epidemiological database). We excluded individuals who were aged <20 years, had systolic hypertension, were taking antihypertensive medication, or had prevalent cardiovascular disease, and studied 1 746 493 individuals (mean age, 42.9±10.7 years; 961 097 men [55.0%]). The average observational period was 1107±855 days. Stage 1 IDH, defined as diastolic BP 80 to 89 mm Hg, and stage 2 IDH, defined as diastolic BP ≥90 mm Hg, were found in 230 513 (13.2%) and 16 159 (0.9%) individuals, respectively. Compared with individuals with normal diastolic BP, individuals with stage 1 and stage 2 IDH were older and more likely to be men. Prevalence of classic risk factors was higher in patients with IDH. Kaplan–Meier curves showed that stage 1 and stage 2 IDH were associated with a higher incidence of cardiovascular events, defined as myocardial infarction, angina pectoris, and stroke. Multivariable analysis showed that stage 1 (hazard ratio [HR], 1.17) and stage 2 (HR, 1.28) IDH were independently associated with a higher incidence of cardiovascular events. Subgroup analyses showed that the association of IDH with cardiovascular events was seen irrespective of age and sex. Conclusions The analysis of a nationwide epidemiological database showed that IDH based on the cutoff value in the 2017 ACC/AHA BP guidelines was associated with an elevated risk of subsequent cardiovascular events.