If the negative association between pre-eclampsia and subsequent breast cancer risk differs by gender, this would strengthen the hypothesis that factors intrinsic to the particular pregnancy may explain the association. The study included 701,006 parous Norwegian women with follow-up for breast cancer through the Cancer Registry of Norway. Breast cancer risk was lower in women with pre-eclampsia/hypertension in their first pregnancy, compared to other women (relative risk, 0.86, 95% CI, 0.78-0.94), after adjustment for age at first birth, maternal birth year, length of gestation, marital status, and parity. The risk reduction was slightly greater if the woman delivered a son as opposed to a daughter (relative risks of 0.79 vs 0.94, P-value for interaction, 0.06), and if pre-eclampsia/hypertension was combined with pre-term delivery, these differences were more pronounced (relative risks, 0.62 vs 1.07, P-value for interaction 0.03). A subanalysis among 176,036 primiparous women showed a substantial risk reduction if the mother delivered a son (relative risk, 0.62, 95% CI, 0.47-0.82), but essentially null if she delivered a daughter (relative risk, 0.92, 95% CI, 0.72-1.18; P-value for interaction, 0.05). These results suggest that the effect of pre-eclampsia/hypertension may be attributed to factors associated with the particular pregnancy rather than an underlying biological trait of the mother. The stronger risk reduction related to having a son suggests a role for sex-dependent hormones in pregnancy.