Migrant and seasonal farmworkers are thought to be at increased risk for occupational injury and illness. Past surveillance efforts that employed medical chart review may not be representative of all farmworkers, since the proportion of farmworkers using migrant health centers (MHCs) and area hospital emergency rooms (ERs) was unknown. The purpose of the current study was to determine the proportion of workers using MHCs versus other sources of occupational health care, and to use these data to correct previous occupational injury and illness rate estimates. Researchers conducted a survey of migrant and seasonal farmworkers in two sites: the Finger Lakes Region of New York and the apple, broccoli, and blueberry regions of Maine. Researchers also conducted MHC and ER medical chart reviews in these regions for comparison purposes. Proportions of occupational morbidity by treatment location were calculated from the survey, and a correction factor was computed to adjust chart review morbidity estimates for Maine and New York State. Among 1103 subjects, 56 work-related injuries were reported: 30 (53.6%) were treated at a MHC, 8 (14.3%) at an ER, 9 (16.1%) at some other location (e.g., home, relative, chiropractor), and 9 (16.1%) were untreated. Mechanisms of injuries treated at MHCs versus all other sources did not differ significantly. The survey-based multiplier (1.87) was applied to previous statewide MHC chart review injury counts from Maine and New York. The corrected injury rates were 7.9 per 100 full-time equivalents (FTE) per year in Maine, and 11.7 per 100 FTE in New York. A chart-review based surveillance system, combined with a correction factor, may provide an effective method of estimating occupational illness and injury rates in this population.