High blood pressure is a risk factor for atherosclerosis in adults, but whether the same is true in children and young people is not known. This is important to guide management of high blood pressure in children and young people. We aimed to investigate the association in children and young people between blood pressure and carotid intima-media thickness, a non-invasive marker of atherosclerosis, through a systematic review. Studies were retrieved from MEDLINE and EMBASE. Articles were eligible for inclusion if they included at least one measurement of blood pressure and at least one measurement of ultrasound-derived carotid intima-media thickness, both measured during childhood (0–19 years), and a measure of effect size or correlation between the two measurements. A total of 3748 studies were identified in the initial search, of which 28 studies were included in this review. The results were mixed, but the largest and highest-quality studies suggested an independent positive association between blood pressure and carotid intima-media thickness in children and young people, even after adjustment for other cardiovascular risk factors. There was no indication of a clear threshold level for the effect of blood pressure on carotid intima-media thickness, hence there are insufficient data to support a pharmacological treatment threshold for the treatment of high blood pressure in children and young people to prevent future cardiovascular disease. The studies included varied widely in terms of quality and design, and it was not possible to combine the data in a meta-analysis. There is likely to be an independent association between blood pressure and carotid intima-media thickness in childhood, but it is not clear at what point this should be treated.