The potential for hormone therapy to cause cancer is the greatest fear for postmenopausal women considering hormone replacement therapy (HRT). Breast cancer is the most common female malignancy, for which HRT is one of many modifiable risk factors, often attracting disproportionate attention. Randomized controlled trials have confirmed that in postmenopausal women aged 50-59 years taking combined oestrogen and progestogen HRT over 5 years, there will be three extra cases of breast cancer per 1000 women. With the use of unopposed conjugated equine oestrogens, there would be four fewer cases over the same time. Women can be advised that the risk of breast cancer is not significantly increased with up to 3 years of combined HRT and up to 5 years of unopposed oestrogen. Unopposed oestrogen increases the risk of endometrial hyperplasia and carcinoma significantly, and this is dose and duration dependent. The addition of progestogen prevents the proliferative effect of oestrogen on the endometrium, and may even reduce the risk of endometrial cancer compared with non-users if given continuously. The use of combined oral contraception in premenopausal women also reduces the risk of endometrial cancer but increases the risk of cervical carcinoma significantly. HRT does not influence the risk of cervical cancer. Epithelial ovarian cancer risk may be slightly increased with long-term use of unopposed oestrogen, is not altered by the addition of progestogen, and is reduced significantly in users of combined oral contraception. The mechanism for these effects is not understood. Colorectal cancer and possibly lung and gastric cancers are reduced by the use of HRT. Apart from a slight increased risk of gallbladder disease and carcinoma with HRT, there are no data linking oestrogen or progestogen with any other malignancies.