Alteration of exposure to ovarian hormones, by eliminating or impairing the function of one or both ovaries, may affect the risk of breast cancer. To assess the relationship between reproductive surgical procedures and breast cancer risk, we conducted a retrospective cohort study involving 524,709 Ontario women who underwent tubal ligation or other salpingectomy, hysterectomy and/or ovariectomy between 1979 and 1993. Data on type and year of surgery were obtained from hospital records and linked with breast cancer diagnoses recorded in the Ontario Cancer Registry. Observed breast cancers were compared with age- and calendar-period expected rates in Ontario. A decreased risk of breast cancer was observed among women who had undergone either hysterectomy (observed/expected [O/E] = 0.87, P < 0.001) or unilateral ovariectomy (O/E = 0.74, P < 0.001). Bilateral ovariectomy and tubal ligation were associated with reduced risk when the surgery occurred before the age of 45 years, and tubal ligation was protective if performed after the age of 54 years. There was no trend in risk with increasing follow-up. Our data support the hypothesis that reduced levels of endogenous oestrogen may be protective for breast cancer in women.