Machining fluids are widely used in a variety of common industrial metalworking operations to lubricate and cool both the tool and the working surfaces. Previous studies have suggested elevated respiratory, digestive, and skin cancers in exposed populations. This cohort study was initiated to assess whether long-term exposure to machining fluids in the course of machining, grinding, and other cutting operations is associated with excess cancer mortality. The cohort includes more than 45,000 automobile production workers from 3 plants, almost 1 million years of follow-up, over 10,000 deaths, and an extensive exposure assessment component. Standardized mortality ratios (SMRs) have been estimated for each of the 3 plants, using both U.S. as well as local populations as reference. Relative risks of 1.2-3.1 have been observed for several specific respiratory and digestive cancers of a priori interest, including cancer of the stomach, large intestine, pancreas, lung, and larynx. In addition, elevated risks for leukemia and asthma were noted. Future exposure-response analyses will provide the opportunity to identify relatively modest excesses in cause-specific mortality risk associated with exposure to specific types (straight, soluble, or synthetic), additives, or components of machining fluids.