Modeled blood lead concentrations derived from EPA's Integrated Exposure Uptake Biokinetic Model LEAD99 (Model) were compared to measured blood lead concentrations for two populations of children living in Colorado mining communities. The study areas were Clear Creek and California Gulch. Both studies presented paired blood lead level measurements and environmental media lead concentrations. The data for this paper were obtained from the Colorado Department of Health and publicly-available submissions made to EPA Region VIII. There was little agreement between model blood lead levels and measured blood lead values. Nine children in the Clear Creek Study were identified as having blood lead levels greater than 10 micrograms per deciliter (μg/dl) by Model or measurement. However, only one individual child was identified as having a blood lead level greater than 10 μg/dl by modelling and measurement. Eight children (89 percent of the children at risk measured) were not identified by the Model. The California Gulch Superfund study included 312 children 294 of which had paired environmental data. The Model estimated that 117 children would have blood lead levels greater than 10 μg/dl. However, the Model only identified 17 of the 26 children that had measured blood lead values greater than 10 μg/dl. The Model failed to identify more than 3 percent of the total population that was at measured risk. The Model did not identify 34.6 percent of the children at measured risk. When the California Gulch populations were grouped by neighborhood, age of housing, and other demographic parameters, the Model predicted lower blood lead concentrations (average of the individual Model runs) than were obtained by measurement. Significant changes have been incorporated into the Model compared to previous versions (Lead 6). The potential impacts of the changes are discussed.