Pesticide exposure may differentially impact young children; they live closer to the ground and take in greater amounts of food relative to body mass than older children or adults. We are using an organophosphate (OP) urinary biomarker screen (gas chromatography with flame photometric detection, GC/FPD) to evaluate pesticide exposure among 154 children < or = 6 years of age living in a heavily farmed border (US-Mexico) community. The screen detects diethylphosphates (DEPs) and dimethylphosphates (DMPs) above a reference range of 1000 non-occupationally exposed individuals (DL=25 microg/g creatinine, Cr). At least one metabolite was detected for 33% of the subjects; many samples contained multiple biomarkers. DEP was detected in 5% of the subjects. DMP and DMTP were frequently measured (25% and 26%, respectively). Biomarker concentrations are adjusted by the body's metabolism of Cr as an indicator of urine dilution. Cr concentrations were examined separately to evaluate their effect on internal dose measures. Cr concentrations were significantly different by season (K-W=0.83, P=0.022). Significant differences exist between the autumn:spring (P=0.038) Cr concentrations and between summer:autumn (P=0.041) Cr concentrations based on Mann-Whitney U=1070.5, z=-2.041, (P=0.041). Our analysis of NHANES III data did not reflect seasonal Cr differences for 6 year olds. No younger children were included. Absorbed daily dose (ADD) estimates were calculated for children with the highest concentrations of metabolite. Calculations are theoretical values assuming that the entirety of a given metabolite was metabolized from a single pesticide. Several class appropriate pesticides were evaluated. For the children with the highest levels, almost all estimated ADDs exceeded the RfD. Although the actual metabolite concentrations dropped appreciably, ADD were still exceeded RfDs at the 95th percentile. The urinary OP screen was effective in identifying subjects with atypical internal doses. Daily Cr yield is a critical component in ADD calculations. Cr variability produces differences in internal dose measurement and estimates of ADD independent of exposure. Cr variability among young children needs to be examined, and caution should be applied when evaluating Cr adjusted internal doses for children.