STUDY OBJECTIVE: The characteristics of some populations make epidemiological measurement extremely difficult. The objective of this study is to identify risk factors that explain variation among incidence densities and proportions of one occupational illness, green tobacco sickness, within one such special population, Latino migrant and seasonal farmworkers in the United States. DESIGN: Prospective cohort study. SETTING: 37 farmworker residential sites located in Granville and Wake Counties, North Carolina, USA. PARTICIPANTS: 182 migrant and seasonal farmworkers that included 178 Latino men, three Latino women, and one non-Hispanic white man. MAIN RESULTS: Green tobacco sickness had a prevalence of 0.082, and an incidence density of events per 100 days of 1.88 among the farmworkers. Prevalence and incidence density increased from early to late agricultural season. Major risk factors included lack of work experience, work activities, and working in wet clothes. Tobacco use was protective. CONCLUSION: Green tobacco sickness has a high incidence among migrant and seasonal farmworkers. Because workers have little control over most risk factors, further research is needed to identify ways to prevent this occupational illness.