Only an estimated 5 percent of rare pediatric diseases have a treatment, although collectively they affect more than ten million children in the US. To stimulate drug development for rare pediatric diseases, Congress expanded the priority review voucher (PRV) program in 2012. A pediatric PRV, which can be sold to another manufacturer, requires the FDA to provide priority six-month review rather than the standard ten-month review to another drug of the company's choosing. We compared rare pediatric disease drugs eligible for a PRV and rare adult disease drugs (which are not eligible for a PRV). We found that compared to drugs for rare adult diseases, drugs for rare pediatric diseases progressed more quickly through all phases of clinical testing and were more likely to be first-in-class. The voucher program was not associated with a change in the rate of new pediatric drugs starting or completing clinical testing, but there was a significant increase in the rate of progress from Phase I to Phase II clinical trials after the program was implemented. New policies may be needed to expand the pipeline of therapies for rare pediatric diseases.