Our objective was to study the preventive effect of lowering blood pressure (BP) by medication and/or lifestyle changes on incident all-cause dementia, Alzheimer's disease and vascular dementia. In this systematic review, we included randomized controlled trials with a BP-lowering intervention. Of the nine included trials, seven assessed the effect of antihypertensive medication and two of a lifestyle or combined intervention. In the intervention arm, 1041 out of 29 029 (3.6%) participants were diagnosed with dementia compared with 1090 out of 28 653 (3.8%) controls during a median follow-up of 3.9 years [range 2–10], resulting in a pooled risk ratio of 0.93 (95% confidence interval 0.84–1.02; I2 16%). Three trials specified dementia subtypes, with no significant effect on Alzheimer's disease or vascular dementia. To conclude, lowering BP by medication and/or lifestyle changes did not lead to a significantly reduced risk of dementia. This appeared independent of dementia subtype.