We analyzed cancer incidence and mortality in a cohort of 22,597 Swedish women who were prescribed replacement hormones. After 13 years of follow-up in national registries, 2,330 incident cancer cases and 848 cancer deaths were observed. Overall, our results were reassuring since incidence rate ratios (SIRs) for 16 cancer sites and mortality ratios (SMRs) for all 10 examined sites were at, or lower than, unity. However, we found that exposure to an estrogen-progestin combined brand was associated with an increasing relative risk of breast cancer with follow-up time, the SIR reaching 1.4 (95% CI 1.1-1.8) after 10 years of follow-up. The relative risk of endometrial cancer was substantially increased, with the highest SIR of 5.0 (95% CI 1.6-5.9) in women prescribed estrogens alone, whereas those given an estrogen-progestin combination showed no elevation in risk. The risk estimates for liver and biliary tract cancers and for colon cancer were reduced by about 40%, notably in women prescribed the estradiol-progestin compound. Further detailed analyses revealed no evidence of adverse or protective effects on the risk of ovarian, uterine cervical, vulvar/vaginal, rectal, pancreatic, renal, lung, thyroid and other endocrine cancers, brain tumors, malignant melanoma or other skin cancers. Hormone replacement therapy was not associated with an increase in mortality for any cancer site, at this time of follow-up. For breast and endometrial cancers, SMRs were below baseline but tended to increase with follow-up time. We conclude that hormone replacement increases the endometrial-cancer risk after unopposed estrogens and the breast-cancer risk-notably after estrogen-progestin combined therapy-and tentatively suggest that it exerts a protective effect against colon and liver cancer risks.