The authors estimated the associations between transient risk factors and laceration injuries in workers at two meatpacking plants in the Midwest.The case-crossover design was used to collect within-subject transient work task and personal-level exposure information. RRs of laceration injuries were estimated by comparing exposures during the 'hazard' period (just before the laceration injury) with exposures in the 'control' period (the previous workweek). Stratified analyses were utilised to estimate the effects of gender, ethnicity, training and the number of adjacent coworkers on each transient risk factor.The authors interviewed 295 meatpacking workers with laceration injuries (mean age 36.6 years, SD 11.2, 75% men, 48% Hispanic). Recent tool sharpening (RR 5.3, 95% CI 3.8 to 7.4) and equipment malfunction (RR 5.3, 95% CI 3.9 to 7.3) were associated with the highest RR for laceration injury, followed by using an unusual work method to accomplish a task (RR 4.1, 95% CI 2.6 to 6.4) and performing an unusual task (RR 2.3, 95% CI 1.8 to 3.0). Rushing and being distracted were not significantly associated with an elevated RR of a laceration injury. In stratified analyses, there were a number of significant differences in laceration risk factors by gender, ethnicity, training, and number of workers on the line.Sharpening tools, equipment malfunction, using an unusual work method to accomplish a task and performing an unusual task were all associated with increased risk of lacerations. Expanded training in atypical work circumstances and evaluation of tool sharpening procedures are intervention areas in meatpacking that need examination.