Although cross-sectional studies found an association between anaemia and dementia, longitudinal studies provided contradictory results. We hypothesize that anaemia might increase the risk of developing dementia because of chronic brain hypo-oxygenation. Using baseline data from a community-based longitudinal study, the Kungsholmen Project, Stockholm, Sweden, we clinically followed 1435 non demented subjects aged 75-95 years for 3 years to detect incident dementia cases (DSM-III-R criteria). Subjects that fulfilled WHO criteria for anaemia, baseline haemoglobin concentration; 130 g/L (men) and 120 g/L (women), had a higher hazard ratios (HR) of developing dementia 3 years later (HR 1.6, 95% CI: 1.1-2.4). In persons with good baseline cognition (MMSE>or=26, n=1139), the association was stronger and still significant after adjustments for conditions potentially related to anaemia and dementia, such as chronic diseases, inflammatory markers, and indicators of nutritional status. The HR was increased even when different haemoglobin cut offs for anaemia definition were used. Thus, anaemia is suggested to be a new potential modifiable risk factor for dementia.