A cluster of Guillaine-Barre syndrome cases in the Vaalharts region, South Africa prompted an investigation of the impact of aerial organophosphate spraying on cholinesterase levels of residents in the region. A prospective study of cholinesterase levels among residents and workers in the area and a control area was performed. Standardized red blood cell cholinesterase levels amongst participants were monitored before (round 1), during (round 2), and after (round 3) the 1996/1997 aerial spraying season. Participants were assigned environmental exposure categories based on the time since (within 10 or 30 days) and distance from (on farm, on neighboring farm, <10 km from farm) aerial pesticide application. There were 342 participants in round 1, of whom 78% participated in round 2, 62% in round 3, and 56% in all three rounds. There was an increase in cholinesterase levels in round 2 (mean increase = 5.96+/-6.25 IU/g hemoglobin) and then a decrease in round 3 (6.17+/-6.51), significantly associated with environmental exposure (participants living on farm or neighboring farm and <10 km from spraying area) controlling for age, gender, alcohol dependence, and usual and recent domestic and occupational pesticide use (e.g., for round 2-round 1 cholinesterase differences, beta; (exposed group relative to unexposed)=5.72+/-1.21 IU/g hemoglobin, P = 0.000, R2 = 0.27, n = 171). The results show a shift in cholinesterase levels associated with residence in the spraying area, but in the direction opposite to that expected from the spraying of pesticides. Seasonal fluctuations in ambient temperature during the study may have influenced the results.