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The contribution of life events (LEs) to breast cancer risk is unclear. The present case-control study of 664 cases and 203 controls demonstrates that adverse events cumulatively increase breast cancer risk and positive events attenuate this increased risk. Assessing LEs on a negative to positive gradient will allow for a more fine-tuned understanding of how personal LEs influence breast cancer risk.The influence of stress on breast cancer risk remains unknown. The goal of the present study was to determine the effect of stress in the form of salient positive and negative valence life events (LEs) on primary invasive breast cancer risk. We hypothesized that salient negative LEs would increase breast cancer risk and salient positive LEs would attenuate this increased risk.We used a case-control design with 664 cases identified through the Cancer Surveillance Program of Orange County and 203 population-based controls. Participants completed a risk factor questionnaire, which included a LE section. Fourteen salient LEs of positive or negative valence were used to quantify stress exposure. A baseline model was constructed, and odds ratios (ORs) were calculated using multivariate unconditional logistic regression.Negative LEs were associated with increased breast cancer risk. The OR for ≥ 4 negative LEs showed a 2.81-fold increase in breast cancer risk (OR, 2.81; 95% confidence interval [CI], 1.47-5.36). A significant dose–response relationship between lifetime negative valence LEs and breast cancer risk was found. Previous personal illness increased breast cancer risk by 3.6-fold (OR, 3.60; 95% CI, 2.50-5.20). In contrast, abortion was associated with a 45% decrease in breast cancer risk (OR, 0.55; 95% CI, 0.34-0.89). Salient positive LEs did not have a significant effect on breast cancer risk. However, they seemed to buffer the adverse effect of salient negative LEs on breast cancer risk.The findings from the present study support the role of salient negative LEs in promoting breast cancer development, with a possible buffering effect of salient positive LEs.