A Cochrane review included 24 studies, but only 16 with a total of 10 114 women had analysable data. Meta-analyses showed no effects of either estrogens only (ERT) or estrogens combined with a progestagen (HRT) on prevention of cognitive impairment after five (OR 1.34, 95% CI 0.95 to 1.9; 1 study, n=2942) and four (OR 1.05, 95% CI 0.72 to 1.54; 1 study, n=4532) years of treatment (trend favouring control in both instances). Analyses assessing the effects of treatment over time found that both ERT and HRT did not maintain or improve cognitive function and may even adversely affect this outcome (WMD = -0.45, 95% CI -0.99 to 0.09, 1 study; WMD = -0.16, 95% CI -0.58 to 0.26, 1 study, respectively at maximum follow up). Negative effects were found for ERT after one year and HRT after three and four years of therapy. Results from smaller trials assessing effects on individual cognitive domains mostly reported no evidence of benefit. There is insufficient evidence to determine whether subgroups of women using specific types of hormone therapy could benefit from treatment.