Prior studies forecasted an incipient shortage of medical oncologists as a result of the aging US population, but the radiation oncology workforce has not been studied. Accordingly, we projected demand for radiation therapy and supply of radiation oncologists in 2010 and 2020 to determine whether a similar shortage may exist for this specialty. Demand for radiation therapy in 2010 and 2020 was estimated by multiplying current radiation utilization rates (as calculated with Surveillance, Epidemiology, and End Results data) by population projections from the Census Bureau. Supply of radiation oncologists was projected using data from the American Board of Radiology inclusive of current radiation oncologists and active residents, accounting for variation in full-time equivalent status and expected survival by age and sex. Between 2010 and 2020, the total number of patients receiving radiation therapy during their initial treatment course is expected to increase by 22%, from 470,000 per year to 575,000 per year. In contrast, assuming that the current graduation rate of 140 residents per year remains constant, the number of full-time equivalent radiation oncologists is expected to increase by only 2%, from 3,943 to 4,022. The size of residency training classes for the years 2014 to 2019 would have to double to 280 residents per year in order for growth in supply of radiation oncologists to equal expected growth in demand. Demand for radiation therapy is expected to grow 10 times faster than supply between 2010 and 2020. Research is needed to explore strategies to enhance capacity to deliver quality radiation therapy despite increased patient loads.