ABSTRACT: As part of healthcare reform, physician assistants (PAs) are needed to help mitigate the physician shortage in the United States. This requires understanding the population of clinically active PAs for accurate prediction purposes. An inventory projection model of PAs drew on historical trends, the PA stock, graduation estimates, retirement trends, and PA intent to retire data. A new source of licensed health professionals, Provider 360 Database, was obtained to augment association information. Program growth and graduate projections indicated an annual 4.7% trend in new entrants to the workforce, offset by annual attrition estimates of 2.9%. As of 2013, there were 84,064 licensed PAs in the United States. The stock and flow equation conservatively predicts the supply of PAs to be 125,847 by 2026. Although the number of clinically active PAs is projected to increase at least by half by 2026, substantial gaps remain in understanding career trends and early attrition influences. Furthermore, education production could be constrained by inadequate clinical training sites and scarcity of faculty.