Age- and sex-related differences in the incidence and prevalence of dementia are well-known. These may be affected by other risk factors including metabolic syndrome or diabetes mellitus, dietary habits and apolipoprotein E genotypes. Furthermore, cognitive function could be modulated by sex hormones such as estrogen and androgen via their anti-oxidative, neuro-protective and vasoprotective effects. Contrary to the results from basic research and observational studies, clinical trials on hormone replacement therapy (HRT) have been controversial. Overall, HRT with estrogen plus progesterone in older women seemed to increase the risk of dementia. In contrast, testosterone replacement in older men and dehydroepiandrosterone (DHEA) replacement older women may have potential protective effects on cognitive function, although clinical evidence is insufficient at present.