Farmworkers can be exposed to a wide variety of cholinesterase inhibiting pesticides. Routine screening for cholinesterase inhibition is not required in many states in the United States, or may be required for workers that apply or handle pesticides, but not those doing routine work in the fields. There is also little information on variations in cholinesterase levels across an agricultural season. Quandt et al. studied cholinesterase levels across the agricultural season (May through August) among 231 migrant farmworkers and explored the association of cholinesterase depression with pesticide exposure. The authors report that average cholinesterase activity levels were lowest in June, with significantly higher mean values in July and August. When adjusted for age, sex, showering after work, and days worked in the fields, the number of organophosphorus and carbamate pesticides detected in urine predicted decreased cholinesterase activity. This study indicates that serial measurements of cholinesterase activity across an agricultural season can detect exposure to pesticides among farmworkers. These results may have implications for the regulation of these chemicals in the agricultural setting.