Women who experience pre-eclampsia or hypertension during pregnancy may have a reduced risk for breast cancer later in life. The evidence is based on case-control studies, and here we report the results of a cohort study exploring the link between pre-eclampsia and gestational hypertension diagnosed in the first pregnancy and subsequent risk for breast cancer. We combined information from the Medical Birth Registry and the Cancer Registry in Norway, which are both nation-wide. Between 1967, when the birth registry was established, and 1998, 694 657 women were recorded with a first birth, and classified according to whether pre-eclampsia and/or hypertension was diagnosed in the first pregnancy. Linkage to the Norwegian Cancer Registry identified 5474 new cases of breast cancer diagnosed subsequently to their first delivery. Compared to other parous women, women with pre-eclampsia and/or hypertension diagnosed in their first pregnancy had 19% lower risk (95% confidence interval, 9 to 29%) for breast cancer, after adjustment for attained age, calendar period of diagnosis, age at first birth, and parity. This result was similar for term and preterm deliveries, across the range of offspring birth weight, and for pre- and postmenopausal women. These results suggest that the pathophysiology surrounding pre-eclampsia and gestational hypertension plays an important role in breast cancer etiology. A better understanding of the underlying processes could provide an insight into the pathogenesis of breast cancer.