This article is a review of blood cholinesterase activity in a cohort of urban pesticide applicators ranging from 1680 to over 3800 workers. During the period 1981-1991, 208, 788 blood samples were taken for measurement of cholinesterase activity with an average of 6 samples per year from each worker. A total of 150 workers or 0.44% of the cohort was removed from exposure to cholinesterase-inhibiting insecticides because of decreased cholinesterase activity. No worker required treatment for signs of cholinesterase inhibition.