Although it is known that farm machinery is a source of serious and catastrophic farm work-related injuries, little is known about the magnitude of, and potential risk factors for, this problem. The study population is from the five-state Regional Rural Injury Study--I (RRIS--I) that included 3,939 farm households and 13,144 persons who were interviewed about their injury experience and farming operation-related exposures during 1990. Rates were calculated for sociodemographic variables and various exposures pertinent to large farm machinery (excluding tractors). Multivariate analyses were conducted using logistic regression, based on a model developed a priori and further confirmed using backward stepwise logistic regression. Among the total farming-related injury events (n = 764), 151 (20%) were related to large machinery use (1,127 injured persons per 100,000 persons per year). Through multivariate analyses, several variables were associated with elevated rate ratios that were important in both models: hours worked per week on the farm (40-59, 60-79, 80+); operation of an auger; field crops as the enterprise requiring the most time; and male gender. In addition, in the backward stepwise model, certain marital status categories (married; separated/widowed/divorced) were also associated with elevated rate ratios that were important. The majority of injury events occurred while persons were lifting, pushing, or pulling (21%), adjusting a machine (20%), or repairing a machine (17%). While only 5% of the cases were hospitalized, 79% required some type of health care. Among all injured persons, 34% were restricted from regular activities for 1 week or more and 19% were restricted for 1 month or more; 25% continued to have persistent problems. In summary, the RRIS-I permitted one of the most comprehensive studies of agricultural machinery-related injuries, to date. The findings indicate that these injuries represent a significant problem, based on the relevant rates, potential risk factors, and consequences from trauma.