Oestrogen therapy has been suggested to have protective effects against Alzheimer's disease. The effects of natural exposure to oestrogen in cognitive disorders have rarely been studied. Assuming that nulliparous women have a higher exposure to natural oestrogen, it could be hypothesised that these women might have a lower risk of Alzheimer's disease than women who have had children. The fertility and number of children in 106 women with a diagnosis of Alzheimer's disease was examined and compared with that of 189 female subjects from two control groups with subjects without dementia. As additional control, the same comparisons were carried out for 40 male patients with Alzheimer's disease and 105 male control subjects. In female subjects, having had children was found to be associated with a diagnosis of Alzheimer's disease. This was not the case in male subjects. The number of children did not seem to affect the risk of Alzheimer's disease, neither in female nor in male subjects. Natural exposure to oestrogen seems to reduce the risk of Alzheimer's disease in women.