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Objective Several pesticides have endocrine disrupting properties, but few studies have examined breast cancer risk in relation to specific active ingredients. Existing studies consider exposure from application or mixing (‘use’), which is less common for female farmers, but none account for exposure from entering sprayed crops to carry out farming tasks (‘re-entry’). This prospective analysis examines exposure to 71 pesticide active ingredients and the risk of breast cancer.Material and Methods We used data from three cohorts participating in an international consortium of agricultural cohorts (AGRICOH): the French Agriculture and Cancer Cohort (AGRICAN, 2005-2017), Cancer in the Norwegian Agricultural Population (1993-2022), and the US Agricultural Health Study (AHS, 1993-2021). The total population includes ~197,000 female farmers and farmers’ wives. Lifetime exposure to specific active ingredients via personal use and re-entry tasks was estimated from crop-exposure matrices, self-report, or both. Cohort-specific adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for breast cancer (overall, pre-, and post-menopausal) will be estimated using Cox regression and combined using random effects meta-analysis.Results Analyses are ongoing. Preliminary results for AHS and AGRICAN indicate 4326 incident breast cancers among the 87,090 combined population. Among the 66 active ingredients examined so far, the median numbers of exposed cases were 51 (IQR: 30-91) in AHS and 21 (IQR: 11-30) in AGRICAN. The most commonly used pesticides in AHS were carbaryl, glyphosate, and malathion, while diazinon and parathion were the most common in AGRICAN.Conclusion This is the largest prospective analysis of specific pesticide active ingredients and breast cancer risk. Strengths include assessment of re-entry exposure, which have largely been ignored in pesticide-cancer analyses, and additional study power from meta-analysis. Cohort heterogeneity presents challenges. Our findings will be relevant to future evaluations of the carcinogenicity of pesticides.