OBJECTIVE: High blood pressure (BP) increases the risk of dementia; however, few studies have reported on the risk of dementia in patients with low-risk, early-grade hypertension. We investigated the protective effect of controlled BP on risk of dementia in treated, low-risk, grade 1 hypertensive patients from the entire National Health Insurance Service National Health Examinee cohort. METHODS: We selected grade 1 hypertension (140-159/90-99 mmHg) patients with low risk, diagnosed in 2005-2006. All patients (N = 128 665) were classified into controlled (average BP < 140/90 mmHg during the follow-up) and uncontrolled (average BP ≥ 140/90 mmHg) BP groups and followed up until 2015. The risk of dementia was estimated using Cox proportional hazard model after adjustments for propensity score. RESULTS: Average BP was 131/81 mmHg in the controlled group (N = 49 408) and 144/87 mmHg in the uncontrolled group (N = 99 257). Overall dementia incidence rates in controlled and uncontrolled groups were 4.9 and 8.1 per 1000 person-year, respectively. The controlled group showed lower risk of overall dementia, Alzheimer's disease, and vascular dementia than the uncontrolled group. The controlled group had a low risk of vascular dementia at all ages, especially in the younger group (age <60). The optimal BP level associated with the lowest risk of dementia was 130 to less than 140 mmHg for SBP and 70 to less than 80 mmHg for DBP. CONCLUSION: We concluded that among even low-risk and grade 1 hypertension patients, controlled BP significantly reduced the risk of dementia, including Alzheimer's disease and vascular dementia.