Exposures to multiple chemical, physical, and biological agents in agricultural work environments can result in confounding that may obscure or distort risks observed in epidemiologic studies. The Agricultural Health Study (AHS) is a large epidemiology study being conducted to investigate health risks among pesticide applicators and their families. During enrollment in the AHS, questionnaires were administered to over 52,000 licensed pesticide applicators from North Carolina and Iowa, who were mostly farmers. Questions about the frequency of various farming tasks were used to estimate the prevalence of exposure to solvents (25%), metals (68%), grain dusts (65%), diesel exhaust fumes (93%), and other hazards, including exposure to pesticides. Most of the farmers in the AHS reported performing routine maintenance tasks at least once a month, such as painting (63%), welding (64%), and repair of pesticide equipment (58%). The majority of farmers (74% in North Carolina; 59% in Iowa) reported holding nonfarm jobs, of which the most frequent were construction and transportation. The majority of the farmers enrolled in the AHS (55%) also reported that they mixed or applied pesticides on 10 or more days per year. The associations between the use of pesticides and the frequency with which the farmers in the AHS reported performing various types of specific farming activities were assessed to evaluate potential confounding. Confounding risk ratios calculated for these activities suggest that the magnitude of bias due to confounding is likely to be minimal.