It is necessary to develop the prevention of Alzheimer's disease, because of the increase in the number of cases and unavailability of a curative treatment. From the data of the cohort PAQUID, we studied the risk of dementia according to leisure activities and the age of cessation of professional activity. The practice of a sport and reading decreases by 25% the risk of dementia during 15 years. The age of cessation of professional activity is not associated with the risk of dementia. An active life seems to be a possible way to prevent dementia.