<i>Objective:</i> To determine the frequency, associated factors and outcome of dementia previous to a stroke. <i>Design:</i> Cross-sectional study of a cohort of 324 consecutive unselected stroke patients (mean age 70.9 years, range 20–98; 255 ischaemic, 46 haemorrhagic and 25 indefinite). <i>Methods:</i> Cognitive and functional status prior to stroke were assessed by means of an interview to a relative, a short version of the Informant Questionnaire on Cognitive Decline in the Elderly and the Barthel Index. The DSM-III-R criteria were used to establish the diagnosis of prestroke dementia. Clinical and CT features of patients with and without prestroke dementia were compared. <i>Results:</i> Forty-nine patients (15%) were demented before stroke; they were significantly older, less well educated, they had more frequently female gender, prior cerebrovascular disease, cerebral and medial temporal lobe atrophy and leukoaraiosis in the CT scan, and they had a higher mortality rate. Female sex (OR 3.7, CI 95% 1.2–12), low education (OR 2.1, CI 95% 1.1–4.2), previous stroke (OR 3.6, CI 95% 1.2–11), and cerebral atrophy (OR 3.8, CI 95% 1.7–8.3) were independently associated with prestroke dementia in the logistic regression analysis. <i>Conclusions:</i> Fifteen percent of stroke patients have prestroke dementia and they have a worse outcome. Factors associated with prestroke dementia are reminiscent both of degenerative and vascular brain pathology.