The advantages and disadvantages of hormone replacement therapy (HRT) have been debated nearly as long as the treatment has been in use, especially the relationship between HRT and risk of cancer development. It is hoped that recently published studies will shed more light on this complex issue. Several large population studies suggest that there may be a small but increased risk of developing breast cancer in HRT users, especially in estrogen and progesterone users. This risk appears most pronounced after 5 years of HRT use. Endometrial cancer, which has long been associated with unopposed estrogen use, can be successfully prevented with the addition of progestins to the HRT regimen, provided it is given for at least 10 days each month. Estrogen replacement therapy has also been shown to significantly reduce the risk for colon cancer but not rectal cancers. Finally, a large prospective study has linked HRT with an increase in ovarian cancer mortality.