The well-described influence of several aspects of reproductive life on the risk for cancer in the reproductive organs has raised concern regarding the safety of exogenous hormones, particularly since sex hormones have become one of the most widely used drugs among women in the western world. The major areas of application include oral contraception and hormone replacement therapy in women with menopausal symptoms. Since the introduction of oral contraceptives onto the Nordic market in the late 1960s, the number of users has grown steadily, to reach proportions of long-term users among women aged 15–45 years in 1985 ranging between 6% (Norway) and 19% (Sweden) and proportions of current users in 1994 ranging between 20% (Norway) and 28% (Sweden). Such data on the current and long-term use of oral contraceptives by the female populations, linked with relative estimates of adverse (cancers of the breast and uterine cervix) and beneficial effects (protection against cancers of the ovary and endometrium), indicate that 95 cases of breast cancer and 40 of cervical cancer will be caused by oral contraceptives annually around 2000 in the Nordic countries, which corresponds to 0.6% of all breast cancers and approximately 3% of all cervical cancers. The beneficial effects include an annual prevention around the year 2000 of approximately 350 cases of ovarian cancer and a similar number of endometrial cancer, for a total about 700 cancer cases annually. The prevalence of long-term users (≥5 years) of hormone replacement therapy among Nordic women aged 40–69 in 1995 was estimated to be 10–11%, which on the basis of an associated relative risk for breast cancer ranging from 1.2–1.5 suggests than an annual total of 260 cases of breast cancer could be avoided in the Nordic countries around the year 2000 if hormone replacement therapy were eliminated. This corresponds to 1.8% of all notified cases of breast cancer among women in these countries.