Background: Post-stroke dementia is defined as any dementia occurring after stroke, and includes vascular, degenerative and mixed dementia. The aim of this study was to assess the incidence of dementia and mild cognitive impairment (MCI) one year after stroke in a population free from pre-stroke cognitive decline, and to investigate the different aetiological subtypes of post-stroke dementia and MCI, using a novel method of subclassification in order to separate vascular causes of MCI or dementia from a neurodegenerative disease. Methods: All patients with a first-ever stroke and TIA admitted to the stroke unit of Asker and Bærum Hospital were invited. After 12 months, dementia and MCI were diagnosed. Sub-classification was made using MRI findings, the results of biomarkers in cerebrospinal fluid and the patients’ clinical cognitive profile. Results: 36 (19.6%) patients developed dementia during the first year after stroke and 69 (37.5%) developed MCI. Fourteen (13.3%) were diagnosed as suffering from degenerative cognitive disease, 34 (32.4%) from vascular cognitive disease, and 57 (54.3%) from mixed disease. Conclusion: Fifty-seven percent suffered from cognitive impairment one year after stroke and only one third from isolated vascular cognitive disease. Post-stroke cognitive impairment is complex with a high coexistence of vascular and degenerative changes.