The aim of the project is to examine the potential role of lifestyle-related factors (e.g., overweight, high blood pressure and inactivity) in middle age for cognitive impairment and dementia in old age. Given the fact that these problems are among the greatest health problems in the older population, knowledge regarding how lifestyle changes can lower the risk is of great practical value. Overweight is one such factor in which the interest has been growing lately. Prospective studies on this matter are needed. A problem with existing studies is that the follow-up period is too short. In the Swedish Twin Registry, on which the present project will be based, information registered in the sixties and seventies concerning the participants lifestyle (e.g., weight, height, occupation, activity, and smoking habit) is available. Thirty to forty years later, the same participants partook in extensive longitudinal studies on cognitive impairment and dementia (OCTO-Twin, SATSA, HARMONY). The information on people's lifestyle during middle age can therefore be used to examine the long-term risk to develop dementia in old age. The extensive follow-up time is one of many aspects that are unique in this project. The project has a great potential to provide knowledge concerning how to prevent cognitive impairment and dementia in old age.