A population-based mail survey of 2,000 farms was conducted to identify rates and patterns of nonfatal agricultural injury in Ontario. Crude, age-adjusted, and stratum-specific rates of farm injury were calculated using data from the returned questionnaires. Patterns of farm injury and its treatment were described by person, place, and time. Response to the survey was 74% (1,364 of 1,842 farms representing 4,110 farm persons). The crude rate of farm injury was 5.8 per 100 persons per year (95% CI: 5.1, 6.5). Common mechanisms of injury included injuries related to the use of farm machinery, overexertion from lifting, accidental falls, and injuries that occurred while working with farm animals. High injury rates were observed in the male 31-40 age group (12.2 per 100 persons per year). Spouses of farm owner-operators (1.7 per 100 persons per year) and their children (2.0 per 100 persons per year) reported lower rates than expected. Most injured persons attributed no factor or "carelessness" as the principle cause of the injury event. Less than 10% of injuries were reported to the provincial workers' compensation board. We conclude that young adult male farmers have the highest rates of injury and warrant targeting by injury control programs. Data from workers' compensation boards have limited utility in the surveillance of most farm injuries in Ontario, in light of their low rate of reporting. The frequency with which inattention or carelessness is attributed to the injury event (as opposed to environmental factors, which might be changed) suggests that injury control programs must address this gap in understanding among farmers, who clearly are vulnerable to traumatic injury.