Pesticide exposure might be a strong risk factor for Parkinson's disease.

We have read with great interest the recent article by Dr Ascherio and coworkers.1 From a large prospective investigation, their results strongly support the hypothesis that exposure to pesticides increases the risk for Parkinson's disease (PD). Given that farmers not exposed to pesticides were not at an increased risk for PD, the high proportion of farmers (76%) included in the study sample has indicated the direct effect of pesticides. However, the PD case-finding procedure has some major pitfalls: self-reported occurrence of PD is likely to lead to a considerable amount of underrecognized and underreported cases of the disease. In fact, the incidence rates that Ascherio and coworkers1 reported were somewhat lower than those previously reported in a population-based study with direct screening of each sampled individual and the clinical confirmation of those who screened positive.2 The authors are concerned about a possible selective underreporting of PD among the nonexposed individuals, with a consequent spurious positive association. On the other hand, in view of the strong and already reported association between pesticides and PD, we think the overlooked PD cases in the exposed population might outnumber those in the nonexposed population. Thus, the risk for PD might be underestimated. In fact, an ecological, population-based, crosssectional study could provide a higher risk ratio.3 We suggest that the results that Dr Ascherio and coworkers acquired through a self-reported mail questionnaire might not reflect accurately the magnitude of the risk or the amount of the disease that can be prevented.