In intracerebral haemorrhage, the most frequent underlying vasculopathies are cerebral amyloid angiopathy and hypertensive vasculopathy. Although both are associated with cognitive impairment, no study has focused on pre-existing dementia in patients with intracerebral haemorrhage. Therefore, we aimed to determine prevalence and mechanisms of pre-existing dementia in a large cohort of patients with an intracerebral haemorrhage. In a cohort of 417 patients, we evaluated the cognitive status before intracerebral haemorrhage with the Informant Questionnaire on Cognitive Decline in the Elderly. The cut-off to diagnose cognitive impairment with no dementia was 53 and 64 for pre-existing dementia. We determined factors associated with pre-existing dementia in multivariate analyses in the overall cohort and among patients with lobar only or deep only intracerebral haemorrhages. We performed post-mortem examinations when possible. Of 417 patients (median age 72 years, interquartile range 58-79), 58 (14%; 95% CI 11-18%) patients had cognitive impairment with no dementia and 65 had pre-existing dementia (16%; 95% CI 12-19%). In lobar intracerebral haemorrhage, the prevalence was 23%, and factors associated with pre-existing dementia were increasing age (odds ratio: 1.09 per year; 95% CI 1.02-1.15), having <8 years of education (odds ratio: 8.37; 95% CI 1.91-36.65) and increasing cortical atrophy (odds ratio: 3.34 per step; 95% CI 1.40-7.96). The five autopsied patients had Alzheimer's disease with cerebral amyloid angiopathy. In deep intracerebral haemorrhage, factors associated with pre-existing dementia were presence of old territorial vascular lesions (odds ratio: 4.52; 95% CI 1.18-17.42) and increasing severity of leucoaraiosis (odds ratio: 4.11 per step; 95% CI 1.73-9.75); the autopsied patient had small-vessel disease without Alzheimer's disease. These findings support the fact that pre-existing dementia is frequent in patients with intracerebral haemorrhage and may be the consequence of two different mechanisms: neurodegeneration with Alzheimer's disease and cerebral amyloid angiopathy in lobar intracerebral haemorrhage versus vascular process in deep intracerebral haemorrhage. These findings may contribute to the improvement of prevention and management of patients with intracerebral haemorrhages.