The Environmental Protection Agency has recognized that organophosphorus pesticides require close regulation and continued monitoring for human health effects and some (e.g chlorpyrifos) have been phased-out from the consumer market due to the special risk that it posed for children. There is growing evidence in support of the association between pesticide exposure and childhood leukemia. Studies of pesticides and their association with childhood cancer have been limited by study designs, self-reporting and lack of biological measurements. While several large studies in California found little evidence of an association between agricultural pesticide use and childhood leukemia, these results are in contrast with the associations observed with household exposures to pesticides. The real association may depend on timing of exposure, type of pesticide, dose and pathway of exposure. Furthermore, some persons may be more susceptible to the effects of specific pesticides due to inherited mutations in their detoxification pathways.