Breast cancer is the leading cancer diagnosed among women and environmental studies have produced few leads on modifiable risk factors. Following an Institute of Medicine recommendation for occupational studies of highly exposed women, we took advantage of an existing cohort of 4503 female hourly autoworkers exposed to metalworking fluid (MWF), complex mixtures of oils and chemicals widely used in metal manufacturing worldwide. Cox proportional hazards models were fit to estimate hazard ratios (HR) for incident breast cancer and cumulative exposure (20 year lag) to straight mineral oils (a known human carcinogen), and water-based soluble and synthetic MWF. Because the state cancer registry began in 1985, decades after the cohort was defined, we restricted analyses to sub-cohorts hired closer to the start of cancer follow-up. Among those hired after 1969, the HR associated with an increase of one interquartile range in straight MWF exposure was 1.13 (95% confidence interval: 1.03, 1.23). In separate analyses of premenopausal breast cancer, as defined by age at diagnosis, the HR was elevated for exposure to synthetic MWF, chemical lubricants with no oil content, suggesting a different mechanism for the younger cases. This study adds to the limited literature regarding quantitative chemical exposures and breast cancer risk.