We examined the relation between the use of hormone replacement therapy (HRT) 
and the incidence of central nervous system (CNS) tumours in a large prospective 
study of 1,147,894 postmenopausal women. Women were aged 56.6 years on average 
at entry, and HRT use was recorded at recruitment and updated, where possible, 
about 3 years later. During a mean follow-up of 5.3 years per woman, 1,266 CNS 
tumours were diagnosed, including 557 gliomas, 311 meningiomas and 117 acoustic 
neuromas. Compared with never users of HRT, the relative risks (RRs) for all 
incident CNS tumours, gliomas, meningiomas and acoustic neuromas in current 
users of HRT were 1.20 (95% CI: 1.05-1.36), 1.09 (95% CI: 0.89-1.32), 1.34 (95% 
CI: 1.03-1.75) and 1.58 (95% CI: 1.02-2.45), respectively, and there was no 
significant difference in the relative risks by tumour type (heterogeneity p = 
0.2). In past users of HRT the relative risk was 1.07 (95% CI: 0.93-1.24) for 
all CNS tumours. Among current users of HRT, there was significant heterogeneity 
by the type of HRT with the users of oestrogen-only HRT at higher risk of all 
CNS tumours than users of oestrogen-progestagen HRT (RR = 1.42, 95% CI: 
1.21-1.67 versus RR = 0.97, 95% CI: 0.82-1.16) (heterogeneity p < 0.001). Among 
current users of oestrogen-only and oestrogen-progestagen HRT, there was no 
significant heterogeneity by duration of use, hormonal constituent or mode of 
administration of HRT.