The size of the cell-telephone only population has increased in recent years raising concerns about the accuracy of RDD-based telephone surveys. Previous research on the impact of cell-telephone only on population estimates has found mixed results. In 2007, the National Immunization Survey (NIS) conducted a pilot study to determine the impact of cell-telephone-only households on childhood vaccination coverage estimates. Th e NIS is a nationwide, list-assisted random digit-dial (RDD) survey to monitor vaccination coverage among children aged 19 to 35 months. 49,300 cell telephone numbers in Illinois were sampled for this pilot. Th is paper will compare vaccination coverage estimates of children in landline households, cell-telephone only households, and combined. We discuss the implications of these results for RDD surveys, and specifi cally for the NIS.